Collapsed Gibbs Sampling for LDA

This is a work in progress implementation of the collapsed Gibbs sampler for the LDA model using the GraphX abstraction primitives.  While this is based on the (non-ergodic) bulk synchronous Gibbs sampler, we do exploit local parameter sharing and if document vertex partitioning is used we recover the Newman et al. style sampler.

Remaining tasks:
- [ ] Unite tests
- [ ] Raw document processing
- [ ] Likelihood calculation (requires log gamma transcendental functions)