Configuration to run coveralls on py33

I setup tox and travis to run pytest-coverage and coveralls on Python 3.3 (no need to run on multiple versions).

Some tests fail, as discussed in #196 (see https://travis-ci.org/davidhalter/jedi/builds/6219522). I don't know Jedi and pytest enough to be able to debug this issue. Maybe @tkf can help?

As soon as the tests work, we can merge it into dev. Sample coverage report: https://coveralls.io/builds/16192

Blocking issues:
- [ ] Failing tests
- [ ] Somehow imports and function/class declarations aren't counted in coverage report: https://coveralls.io/builds/16192/source?filename=jedi%2Fevaluate_representation.py