Play nice with Python 3.2+.

- Re-work requirements to install cleanly with Python 3.2+.
- Adjust defaults to use thriftpy_sync worker on Python 3.2+.
- Update thriftpy_worker.py to conditionally declare GeventThriftpyWorker
  only if gevent is available.
- Added section explaining Python 3 support.
- Added docs for using the thriftpy workers.
- Added sample thriftpy_app.py in tests/.
- Updated setup.py to handle test requirements on py27 and py32+ separately.