Python 3 support

As promised, here's a pull request which adds support for Python 3.  I have modified the tox.ini file to test against Python 3.3 (I have not tested with 3.0 or 3.1 or 3.2 though it should work with those as well).  I also configured tox to test against Python 2.5, since you mentioned you had customers using 2.5.  The unit tests seem to work with 2.5, 2.6, 2.7, pypy, and 3.3, and I tested using my account to send a test charge with 2.5 and 3.3 and it worked in both cases.

The most significant change I made was to move the test directory to live within the actual stripe package itself.  I didn't want to have to do this; I also like keeping unit tests separate when possible instead of bundling them as a sub-package.  However, I had to do this so that tox would run tests against the proper 2to3 source code, as a workaround for this unresolved tox issue: https://code.google.com/p/pytox/issues/detail?id=21

As indicated, I'm using 2to3 in setup.py, so you can still just have one single source code base which is automatically converted on install.  Let me know if you have any questions - I'll be using this for my own Python 3 project, which is in the process of switching over from Paypal to Stripe at the moment, so I'll let you know if I run into any problems.