Add riemann_pb2 using https://github.com/GreatFruitOmsk/protobuf-py3
Python 3 support (thanks @Lupino, closes #15)

* Changed riemann_client.riemann_pb2 to wrap py2 and py3 modules
* Changed setup.py to dynamically select a protobuf dependency

* Docs use version and author from module
* Tests updated to work on Python 3
* Tests run on py33 and py34
* Tests run coverage and flake8 under py26 and py34