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/.
* Added notes for defaults on 2.7 and 3.2+.
* Updated setup.py to handle test requirements on py27 and py32+ separately.

A couple notes:

* Based on some poking around, I gathered this targets Python 2.7 exclusively, so I enforced that along with all the version checking for Python 3. If that was an bad assumption, please don't be shy. :smile: 
* Tests **do not pass** on Python 3.2+. I'm looking for feedback on how best to approach it given how the tests are fairly intertwined with gevent.