Add a tox.ini

The good news is that tests pass for all of these environments!

```
~/dev/git-repos/virtualenv$ tox
_________________________________ [tox sdist] __________________________________
[TOX] ***creating sdist package
[TOX] /Users/marca/dev/git-repos/virtualenv$ /Users/marca/.pythonbrew/pythons/Python-2.7.2/bin/python setup.py sdist --formats=zip --dist-dir .tox/dist >.tox/log/0.log
[TOX] ***copying new sdistfile to '/Users/marca/.tox/distshare/virtualenv-1.7.1.2.post1.zip'
______________________________ [tox testenv:py25] ______________________________
[TOX] ***reusing existing matching virtualenv py25
[TOX] ***upgrade-installing sdist
[TOX] /Users/marca/dev/git-repos/virtualenv/.tox/py25/log$ ../bin/pip install --download-cache=/Users/marca/dev/git-repos/virtualenv/.tox/_download /Users/marca/dev/git-repos/virtualenv/.tox/dist/virtualenv-1.7.1.2.post1.zip -U --no-deps >11.log
[TOX] /Users/marca/dev/git-repos/virtualenv$ .tox/py25/bin/python setup.py test
running test
running egg_info
writing virtualenv.egg-info/PKG-INFO
writing top-level names to virtualenv.egg-info/top_level.txt
writing dependency_links to virtualenv.egg-info/dependency_links.txt
writing entry points to virtualenv.egg-info/entry_points.txt
reading manifest file 'virtualenv.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
warning: no previously-included files matching '*.*' found under directory 'docs/_templates'
writing manifest file 'virtualenv.egg-info/SOURCES.txt'
running build_ext
Should have a version string ... ok
Should return absolute path if given and exists ... ok
Should exit when with absolute path if not exists ... ok
Should exit when with absolute path if not exists ... ok

----------------------------------------------------------------------
Ran 4 tests in 0.049s

OK
______________________________ [tox testenv:py26] ______________________________
[TOX] ***reusing existing matching virtualenv py26
[TOX] ***upgrade-installing sdist
[TOX] /Users/marca/dev/git-repos/virtualenv/.tox/py26/log$ ../bin/pip install --download-cache=/Users/marca/dev/git-repos/virtualenv/.tox/_download /Users/marca/dev/git-repos/virtualenv/.tox/dist/virtualenv-1.7.1.2.post1.zip -U --no-deps >12.log
[TOX] /Users/marca/dev/git-repos/virtualenv$ .tox/py26/bin/python setup.py test
running test
running egg_info
writing virtualenv.egg-info/PKG-INFO
writing top-level names to virtualenv.egg-info/top_level.txt
writing dependency_links to virtualenv.egg-info/dependency_links.txt
writing entry points to virtualenv.egg-info/entry_points.txt
reading manifest file 'virtualenv.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
warning: no previously-included files matching '*.*' found under directory 'docs/_templates'
writing manifest file 'virtualenv.egg-info/SOURCES.txt'
running build_ext
Should have a version string ... ok
Should return absolute path if given and exists ... ok
Should exit when with absolute path if not exists ... ok
Should exit when with absolute path if not exists ... ok

----------------------------------------------------------------------
Ran 4 tests in 0.049s

OK
______________________________ [tox testenv:py27] ______________________________
[TOX] ***reusing existing matching virtualenv py27
[TOX] ***upgrade-installing sdist
[TOX] /Users/marca/dev/git-repos/virtualenv/.tox/py27/log$ ../bin/pip install --download-cache=/Users/marca/dev/git-repos/virtualenv/.tox/_download /Users/marca/dev/git-repos/virtualenv/.tox/dist/virtualenv-1.7.1.2.post1.zip -U --no-deps >12.log
[TOX] /Users/marca/dev/git-repos/virtualenv$ .tox/py27/bin/python setup.py test
running test
running egg_info
writing virtualenv.egg-info/PKG-INFO
writing top-level names to virtualenv.egg-info/top_level.txt
writing dependency_links to virtualenv.egg-info/dependency_links.txt
writing entry points to virtualenv.egg-info/entry_points.txt
reading manifest file 'virtualenv.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
warning: no previously-included files matching '*.*' found under directory 'docs/_templates'
writing manifest file 'virtualenv.egg-info/SOURCES.txt'
running build_ext
tests.test_virtualenv.test_version
Should have a version string ... ok
tests.test_virtualenv.test_resolve_interpreter_with_absolute_path
Should return absolute path if given and exists ... ok
tests.test_virtualenv.test_resolve_intepreter_with_nonexistant_interpreter
Should exit when with absolute path if not exists ... ok
tests.test_virtualenv.test_resolve_intepreter_with_invalid_interpreter
Should exit when with absolute path if not exists ... ok

----------------------------------------------------------------------
Ran 4 tests in 0.038s

OK
______________________________ [tox testenv:py31] ______________________________
[TOX] ***reusing existing matching virtualenv py31
[TOX] ***upgrade-installing sdist
[TOX] /Users/marca/dev/git-repos/virtualenv/.tox/py31/log$ ../bin/pip install --download-cache=/Users/marca/dev/git-repos/virtualenv/.tox/_download /Users/marca/dev/git-repos/virtualenv/.tox/dist/virtualenv-1.7.1.2.post1.zip -U --no-deps >12.log
[TOX] /Users/marca/dev/git-repos/virtualenv$ .tox/py31/bin/python setup.py test
running test
running egg_info
writing virtualenv.egg-info/PKG-INFO
writing top-level names to virtualenv.egg-info/top_level.txt
writing dependency_links to virtualenv.egg-info/dependency_links.txt
writing entry points to virtualenv.egg-info/entry_points.txt
reading manifest file 'virtualenv.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
warning: no previously-included files matching '*.*' found under directory 'docs/_templates'
writing manifest file 'virtualenv.egg-info/SOURCES.txt'
running build_ext
Should have a version string ... ok
Should exit when with absolute path if not exists ... ok
Should exit when with absolute path if not exists ... ok
Should return absolute path if given and exists ... ok

----------------------------------------------------------------------
Ran 4 tests in 0.043s

OK
______________________________ [tox testenv:py32] ______________________________
[TOX] ***reusing existing matching virtualenv py32
[TOX] ***upgrade-installing sdist
[TOX] /Users/marca/dev/git-repos/virtualenv/.tox/py32/log$ ../bin/pip install --download-cache=/Users/marca/dev/git-repos/virtualenv/.tox/_download /Users/marca/dev/git-repos/virtualenv/.tox/dist/virtualenv-1.7.1.2.post1.zip -U --no-deps >12.log
[TOX] /Users/marca/dev/git-repos/virtualenv$ .tox/py32/bin/python setup.py test
running test
running egg_info
writing virtualenv.egg-info/PKG-INFO
writing top-level names to virtualenv.egg-info/top_level.txt
writing dependency_links to virtualenv.egg-info/dependency_links.txt
writing entry points to virtualenv.egg-info/entry_points.txt
reading manifest file 'virtualenv.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
warning: no previously-included files matching '*.*' found under directory 'docs/_templates'
writing manifest file 'virtualenv.egg-info/SOURCES.txt'
running build_ext
tests.test_virtualenv.test_version
Should have a version string ... ok
tests.test_virtualenv.test_resolve_intepreter_with_invalid_interpreter
Should exit when with absolute path if not exists ... ok
tests.test_virtualenv.test_resolve_intepreter_with_nonexistant_interpreter
Should exit when with absolute path if not exists ... ok
tests.test_virtualenv.test_resolve_interpreter_with_absolute_path
Should return absolute path if given and exists ... ok

----------------------------------------------------------------------
Ran 4 tests in 0.018s

OK
______________________________ [tox testenv:pypy] ______________________________
[TOX] ***reusing existing matching virtualenv pypy
[TOX] ***upgrade-installing sdist
[TOX] /Users/marca/dev/git-repos/virtualenv/.tox/pypy/log$ ../bin/pip install --download-cache=/Users/marca/dev/git-repos/virtualenv/.tox/_download /Users/marca/dev/git-repos/virtualenv/.tox/dist/virtualenv-1.7.1.2.post1.zip -U --no-deps >12.log
[TOX] /Users/marca/dev/git-repos/virtualenv$ .tox/pypy/bin/python setup.py test
running test
running egg_info
writing top-level names to virtualenv.egg-info/top_level.txt
writing entry points to virtualenv.egg-info/entry_points.txt
writing virtualenv.egg-info/PKG-INFO
writing dependency_links to virtualenv.egg-info/dependency_links.txt
reading manifest file 'virtualenv.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
warning: no previously-included files matching '*.*' found under directory 'docs/_templates'
writing manifest file 'virtualenv.egg-info/SOURCES.txt'
running build_ext
tests.test_virtualenv.test_version
Should have a version string ... ok
tests.test_virtualenv.test_resolve_interpreter_with_absolute_path
Should return absolute path if given and exists ... ok
tests.test_virtualenv.test_resolve_intepreter_with_nonexistant_interpreter
Should exit when with absolute path if not exists ... ok
tests.test_virtualenv.test_resolve_intepreter_with_invalid_interpreter
Should exit when with absolute path if not exists ... ok

----------------------------------------------------------------------
Ran 4 tests in 0.147s

OK
_____________________________ [tox testenv:jython] _____________________________
[TOX] ***reusing existing matching virtualenv jython
[TOX] ***upgrade-installing sdist
[TOX] /Users/marca/dev/git-repos/virtualenv/.tox/jython/log$ ../bin/pip install --download-cache=/Users/marca/dev/git-repos/virtualenv/.tox/_download /Users/marca/dev/git-repos/virtualenv/.tox/dist/virtualenv-1.7.1.2.post1.zip -U --no-deps >5.log
[TOX] WARNING:test command found but not installed in testenv
  cmd: /Users/marca/dev/git-repos/virtualenv/env/bin/python
  env: /Users/marca/dev/git-repos/virtualenv/.tox/jython
Maybe forgot to specify a dependency?
[TOX] /Users/marca/dev/git-repos/virtualenv$ /Users/marca/dev/git-repos/virtualenv/env/bin/python setup.py test
running test
running egg_info
writing virtualenv.egg-info/PKG-INFO
writing top-level names to virtualenv.egg-info/top_level.txt
writing dependency_links to virtualenv.egg-info/dependency_links.txt
writing entry points to virtualenv.egg-info/entry_points.txt
reading manifest file 'virtualenv.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
warning: no previously-included files matching '*.*' found under directory 'docs/_templates'
writing manifest file 'virtualenv.egg-info/SOURCES.txt'
running build_ext
tests.test_virtualenv.test_version
Should have a version string ... ok
tests.test_virtualenv.test_resolve_interpreter_with_absolute_path
Should return absolute path if given and exists ... ok
tests.test_virtualenv.test_resolve_intepreter_with_nonexistant_interpreter
Should exit when with absolute path if not exists ... ok
tests.test_virtualenv.test_resolve_intepreter_with_invalid_interpreter
Should exit when with absolute path if not exists ... ok

----------------------------------------------------------------------
Ran 4 tests in 0.038s

OK
________________________________ [tox summary] _________________________________
[TOX] py25: commands succeeded
[TOX] py26: commands succeeded
[TOX] py27: commands succeeded
[TOX] py31: commands succeeded
[TOX] py32: commands succeeded
[TOX] pypy: commands succeeded
[TOX] jython: commands succeeded
[TOX] congratulations :)
```