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

[msabramo]

Passing Travis build: http://travis-ci.org/#!/msabramo/When.py/builds/1763857

Sample tox output:

~/dev/git-repos/When.py$ tox
GLOB sdist-make: /Users/marca/dev/git-repos/When.py/setup.py
py26 sdist-reinst: /Users/marca/dev/git-repos/When.py/.tox/dist/whenpy-0.3.0.zip
py26 runtests: commands[0]
WARNING:test command found but not installed in testenv
  cmd: /usr/bin/make
  env: /Users/marca/dev/git-repos/When.py/.tox/py26
Maybe forgot to specify a dependency?
nosetests ./tests/*
..................................
----------------------------------------------------------------------
Ran 34 tests in 0.015s

OK
py27 sdist-reinst: /Users/marca/dev/git-repos/When.py/.tox/dist/whenpy-0.3.0.zip
py27 runtests: commands[0]
WARNING:test command found but not installed in testenv
  cmd: /usr/bin/make
  env: /Users/marca/dev/git-repos/When.py/.tox/py27
Maybe forgot to specify a dependency?
nosetests ./tests/*
..................................
----------------------------------------------------------------------
Ran 34 tests in 0.016s

OK
py32 sdist-reinst: /Users/marca/dev/git-repos/When.py/.tox/dist/whenpy-0.3.0.zip
py32 runtests: commands[0]
WARNING:test command found but not installed in testenv
  cmd: /usr/bin/make
  env: /Users/marca/dev/git-repos/When.py/.tox/py32
Maybe forgot to specify a dependency?
nosetests ./tests/*
..................................
----------------------------------------------------------------------
Ran 34 tests in 0.015s

OK
py33 sdist-reinst: /Users/marca/dev/git-repos/When.py/.tox/dist/whenpy-0.3.0.zip
py33 runtests: commands[0]
WARNING:test command found but not installed in testenv
  cmd: /usr/bin/make
  env: /Users/marca/dev/git-repos/When.py/.tox/py33
Maybe forgot to specify a dependency?
nosetests ./tests/*
..................................
----------------------------------------------------------------------
Ran 34 tests in 0.017s

OK
_________________________________________________________________________ summary __________________________________________________________________________
  py26: commands succeeded
  py27: commands succeeded
  py32: commands succeeded
  py33: commands succeeded
  congratulations :)