Update the project layout

This PR does several things concerning the layout of the MaxCDN project.  This should help make development and testing a bit easier.

1. Renamed the `test` directory to `tests` to avoid potential conflicts with the builtin `test` module
2. Added [Tox](http://tox.readthedocs.org/en/latest/) for testing against various Python interpreters
3. Forced the usage of `virtualenv` so people running tests don't have to globally install packages (other than `virtualenv`)
4. Updated and simplified the Makefile to make use of `virtualenv` and `tox`
5. Clean up `setup.py` and add classifiers