Python 3 compatibility, and updated documentation

- Fixed Python 3 compatibility issues
    - Tests pass on Python 2.6, 2.7, 3.3, 3.4, and PyPy, with Django 1.4, 1.5, and 1.6.
    - This required sacrificing compatibility with Django 1.3 (it is no longer supported)
- Re-arranged `test_project`, making it possible to run tests without real Twilio credentials
    - Travis testing works now!
- Added `test_app.utils.TwilioRequestFactory` to generate requests with auto-computed Twilio signatures.
    - Tests are a lot simpler now.
- Removed `run_tests.py` in favor of `manage.py test`
    - This is the norm in Django projects.
    - `make test` still works
- Separated requirements into production (`requirements.txt`) and testing (`test_requirements.txt`)
- Updated documentation with all these changes (and fixed a few typos) :)
- Bumped version to `0.7`