py.test-based test runner

- Nose dependency is removed in favor of [py.test](http://pytest.org/latest/)
- Dependencies are installed via pip+requirements.txt
- junit xml filename changed to `results.xml` from `nosetests.xml`
- python-based test runner
- `tests/unit/py` renamed to `tests/unit/py2` and `tests/external/py` renamed to `tests/external/py2` for python namespace conflict resolution