Implement mixture api for PitmanYor clustering model
Add cython wrapper for PitmanYor Mixture api
Refactor test_clustering.py
Add global has_cython flag for safer testing
Merge branch 'master' into mixture

Conflicts:
	distributions/tests/test_clustering.py
Link clustering.cc to python module when needed.
Merge branch 'master' into mixture
Get PitmanYor::Mixture working
Allow C++ models to propagate exceptions to python
Merge branch 'master' into loom
Add protobuf to libraries in setup.py