Add support for tox

```
$ tox
GLOB sdist-make: /Users/marca/dev/git-repos/gmemcache/setup.py
py26 inst-nodeps: /Users/marca/dev/git-repos/gmemcache/.tox/dist/gmemcache-0.0.1.zip
py26 runtests: commands[0] | nosetests
........
----------------------------------------------------------------------
Ran 8 tests in 4.210s

OK
py27 inst-nodeps: /Users/marca/dev/git-repos/gmemcache/.tox/dist/gmemcache-0.0.1.zip
py27 runtests: commands[0] | nosetests
........
----------------------------------------------------------------------
Ran 8 tests in 4.196s

OK
py33 inst-nodeps: /Users/marca/dev/git-repos/gmemcache/.tox/dist/gmemcache-0.0.1.zip
py33 runtests: commands[0] | nosetests
EE
======================================================================
ERROR: Failure: ImportError (cannot import name MemcacheConnection)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/marca/dev/git-repos/gmemcache/.tox/py33/lib/python3.3/site-packages/nose/failure.py", line 38, in runTest
    raise self.exc_val.with_traceback(self.tb)
  File "/Users/marca/dev/git-repos/gmemcache/.tox/py33/lib/python3.3/site-packages/nose/loader.py", line 413, in loadTestsFromName
    addr.filename, addr.module)
  File "/Users/marca/dev/git-repos/gmemcache/.tox/py33/lib/python3.3/site-packages/nose/importer.py", line 47, in importFromPath
    return self.importFromDir(dir_path, fqname)
  File "/Users/marca/dev/git-repos/gmemcache/.tox/py33/lib/python3.3/site-packages/nose/importer.py", line 94, in importFromDir
    mod = load_module(part_fqname, fh, filename, desc)
  File "/Users/marca/dev/git-repos/gmemcache/.tox/py33/lib/python3.3/imp.py", line 190, in load_module
    return load_package(name, filename)
  File "/Users/marca/dev/git-repos/gmemcache/.tox/py33/lib/python3.3/imp.py", line 160, in load_package
    return _bootstrap.SourceFileLoader(name, path).load_module(name)
  File "<frozen importlib._bootstrap>", line 584, in _check_name_wrapper
  File "<frozen importlib._bootstrap>", line 1022, in load_module
  File "<frozen importlib._bootstrap>", line 1003, in load_module
  File "<frozen importlib._bootstrap>", line 560, in module_for_loader_wrapper
  File "<frozen importlib._bootstrap>", line 868, in _load_module
  File "<frozen importlib._bootstrap>", line 313, in _call_with_frames_removed
  File "/Users/marca/dev/git-repos/gmemcache/gmemcache/__init__.py", line 3, in <module>
    from gmemcache import MemcacheConnection, MemcachePoolConnection
ImportError: cannot import name MemcacheConnection

======================================================================
ERROR: Failure: ImportError (cannot import name MemcacheConnection)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/marca/dev/git-repos/gmemcache/.tox/py33/lib/python3.3/site-packages/nose/failure.py", line 38, in runTest
    raise self.exc_val.with_traceback(self.tb)
  File "/Users/marca/dev/git-repos/gmemcache/.tox/py33/lib/python3.3/site-packages/nose/loader.py", line 413, in loadTestsFromName
    addr.filename, addr.module)
  File "/Users/marca/dev/git-repos/gmemcache/.tox/py33/lib/python3.3/site-packages/nose/importer.py", line 47, in importFromPath
    return self.importFromDir(dir_path, fqname)
  File "/Users/marca/dev/git-repos/gmemcache/.tox/py33/lib/python3.3/site-packages/nose/importer.py", line 94, in importFromDir
    mod = load_module(part_fqname, fh, filename, desc)
  File "/Users/marca/dev/git-repos/gmemcache/.tox/py33/lib/python3.3/imp.py", line 180, in load_module
    return load_source(name, filename, file)
  File "/Users/marca/dev/git-repos/gmemcache/.tox/py33/lib/python3.3/imp.py", line 119, in load_source
    _LoadSourceCompatibility(name, pathname, file).load_module(name)
  File "<frozen importlib._bootstrap>", line 584, in _check_name_wrapper
  File "<frozen importlib._bootstrap>", line 1022, in load_module
  File "<frozen importlib._bootstrap>", line 1003, in load_module
  File "<frozen importlib._bootstrap>", line 560, in module_for_loader_wrapper
  File "<frozen importlib._bootstrap>", line 868, in _load_module
  File "<frozen importlib._bootstrap>", line 313, in _call_with_frames_removed
  File "/Users/marca/dev/git-repos/gmemcache/tests/test_gmemcache.py", line 6, in <module>
    from gmemcache import MemcacheConnection
  File "/Users/marca/dev/git-repos/gmemcache/gmemcache/__init__.py", line 3, in <module>
    from gmemcache import MemcacheConnection, MemcachePoolConnection
ImportError: cannot import name MemcacheConnection

----------------------------------------------------------------------
Ran 2 tests in 0.063s

FAILED (errors=2)
ERROR: InvocationError: '/Users/marca/dev/git-repos/gmemcache/.tox/py33/bin/nosetests'
___________________________________________________________________________________ summary ____________________________________________________________________________________
  py26: commands succeeded
  py27: commands succeeded
ERROR:   py33: commands failed
```