Add support for tox (http://tox.testrun.org/)

[Tox](http://tox.testrun.org/) is a nice tool for testing against multiple versions of Python.

Cool. Your module works on every Python I tried! :-)

```
~/dev/git-repos/python-reloader$ tox
_________________________________ [tox sdist] __________________________________
[TOX] ***creating sdist package
[TOX] /Users/marca/dev/git-repos/python-reloader$ /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/reloader-0.3.zip'
______________________________ [tox testenv:py26] ______________________________
[TOX] ***reusing existing matching virtualenv py26
[TOX] ***upgrade-installing sdist
[TOX] /Users/marca/dev/git-repos/python-reloader/.tox/py26/log$ ../bin/pip install --download-cache=/Users/marca/dev/git-repos/python-reloader/.tox/_download /Users/marca/dev/git-repos/python-reloader/.tox/dist/reloader-0.3.zip -U --no-deps >7.log
[TOX] /Users/marca/dev/git-repos/python-reloader$ .tox/py26/bin/python setup.py test
running test
running egg_info
writing reloader.egg-info/PKG-INFO
writing top-level names to reloader.egg-info/top_level.txt
writing dependency_links to reloader.egg-info/dependency_links.txt
reading manifest file 'reloader.egg-info/SOURCES.txt'
writing manifest file 'reloader.egg-info/SOURCES.txt'
running build_ext
test_import (tests.test_reloader.ReloaderTest) ... ok
test_reload (tests.test_reloader.ReloaderTest) ... ok

----------------------------------------------------------------------
Ran 2 tests in 0.001s

OK
______________________________ [tox testenv:py27] ______________________________
[TOX] ***reusing existing matching virtualenv py27
[TOX] ***upgrade-installing sdist
[TOX] /Users/marca/dev/git-repos/python-reloader/.tox/py27/log$ ../bin/pip install --download-cache=/Users/marca/dev/git-repos/python-reloader/.tox/_download /Users/marca/dev/git-repos/python-reloader/.tox/dist/reloader-0.3.zip -U --no-deps >7.log
[TOX] /Users/marca/dev/git-repos/python-reloader$ .tox/py27/bin/python setup.py test
running test
running egg_info
writing reloader.egg-info/PKG-INFO
writing top-level names to reloader.egg-info/top_level.txt
writing dependency_links to reloader.egg-info/dependency_links.txt
reading manifest file 'reloader.egg-info/SOURCES.txt'
writing manifest file 'reloader.egg-info/SOURCES.txt'
running build_ext
test_import (tests.test_reloader.ReloaderTest) ... ok
test_reload (tests.test_reloader.ReloaderTest) ... ok

----------------------------------------------------------------------
Ran 2 tests in 0.001s

OK
______________________________ [tox testenv:py31] ______________________________
[TOX] ***reusing existing matching virtualenv py31
[TOX] ***upgrade-installing sdist
[TOX] /Users/marca/dev/git-repos/python-reloader/.tox/py31/log$ ../bin/pip install --download-cache=/Users/marca/dev/git-repos/python-reloader/.tox/_download /Users/marca/dev/git-repos/python-reloader/.tox/dist/reloader-0.3.zip -U --no-deps >3.log
[TOX] /Users/marca/dev/git-repos/python-reloader$ .tox/py31/bin/python setup.py test
running test
running egg_info
writing reloader.egg-info/PKG-INFO
writing top-level names to reloader.egg-info/top_level.txt
writing dependency_links to reloader.egg-info/dependency_links.txt
reading manifest file 'reloader.egg-info/SOURCES.txt'
writing manifest file 'reloader.egg-info/SOURCES.txt'
running build_ext
test_import (tests.test_reloader.ReloaderTest) ... ok
test_reload (tests.test_reloader.ReloaderTest) ... ok

----------------------------------------------------------------------
Ran 2 tests in 0.002s

OK
______________________________ [tox testenv:py32] ______________________________
[TOX] ***reusing existing matching virtualenv py32
[TOX] ***upgrade-installing sdist
[TOX] /Users/marca/dev/git-repos/python-reloader/.tox/py32/log$ ../bin/pip install --download-cache=/Users/marca/dev/git-repos/python-reloader/.tox/_download /Users/marca/dev/git-repos/python-reloader/.tox/dist/reloader-0.3.zip -U --no-deps >7.log
[TOX] /Users/marca/dev/git-repos/python-reloader$ .tox/py32/bin/python setup.py test
running test
running egg_info
writing reloader.egg-info/PKG-INFO
writing top-level names to reloader.egg-info/top_level.txt
writing dependency_links to reloader.egg-info/dependency_links.txt
reading manifest file 'reloader.egg-info/SOURCES.txt'
writing manifest file 'reloader.egg-info/SOURCES.txt'
running build_ext
test_import (tests.test_reloader.ReloaderTest) ... ok
test_reload (tests.test_reloader.ReloaderTest) ... ok

----------------------------------------------------------------------
Ran 2 tests in 0.002s

OK
______________________________ [tox testenv:pypy] ______________________________
[TOX] ***reusing existing matching virtualenv pypy
[TOX] ***upgrade-installing sdist
[TOX] /Users/marca/dev/git-repos/python-reloader/.tox/pypy/log$ ../bin/pip install --download-cache=/Users/marca/dev/git-repos/python-reloader/.tox/_download /Users/marca/dev/git-repos/python-reloader/.tox/dist/reloader-0.3.zip -U --no-deps >7.log
[TOX] /Users/marca/dev/git-repos/python-reloader$ .tox/pypy/bin/python setup.py test
running test
running egg_info
writing top-level names to reloader.egg-info/top_level.txt
writing reloader.egg-info/PKG-INFO
writing dependency_links to reloader.egg-info/dependency_links.txt
reading manifest file 'reloader.egg-info/SOURCES.txt'
writing manifest file 'reloader.egg-info/SOURCES.txt'
running build_ext
test_import (tests.test_reloader.ReloaderTest) ... ok
test_reload (tests.test_reloader.ReloaderTest) ... ok

----------------------------------------------------------------------
Ran 2 tests in 0.002s

OK
________________________________ [tox summary] _________________________________
[TOX] py26: commands succeeded
[TOX] py27: commands succeeded
[TOX] py31: commands succeeded
[TOX] py32: commands succeeded
[TOX] pypy: commands succeeded
[TOX] congratulations :)
```