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

[msabramo]

Sample tox output:

~/dev/git-repos/sphinx-git$ tox
GLOB sdist-make: /Users/marca/dev/git-repos/sphinx-git/setup.py
py26 sdist-reinst: /Users/marca/dev/git-repos/sphinx-git/.tox/dist/sphinx-git-1.zip
py26 runtests: commands[0]
py26 runtests: commands[1]
......
----------------------------------------------------------------------
Ran 6 tests in 1.418s

OK
py27 sdist-reinst: /Users/marca/dev/git-repos/sphinx-git/.tox/dist/sphinx-git-1.zip
py27 runtests: commands[0]
py27 runtests: commands[1]
......
----------------------------------------------------------------------
Ran 6 tests in 8.884s

OK
_________________________________________________________________________ summary __________________________________________________________________________
  py26: commands succeeded
  py27: commands succeeded
  congratulations :)

[OddBloke]

Merged. Thanks!