Add tox support

- Add a tox ini file
- Add tox.ini to Manifest.in
- Describe tox in docs
- Allow passing arguments to py.test by tox -- py-test-args
- Various additions and configurations to tox.ini
- Add a lot of useful python versions as default envs
- Add skip_missing_interpreters=True as a useful default
- Add two environments for pyflakes and pep8 tests
- Add changedir directives for tests
- Add pytest dependencies
- Improve documentation