Re-implementation of collapsed gibbs sampler handlers + cleaning

* re-implementation of collapsed gibbs sampler without handlers, based on sampling from factors only
* removal of Categorical Factors, because they can all be represented by Discrete Factors using DiscreteVar as child and/or parent type
* resolves issue #178
* small tests were conducted for training LDA and it worked
* still a little bit slow compared to the efficient LDA implementation, but at least as fast as previous implementation of collapsed gibbs sampling
Are the Lexicon changes integral to the GibbsSampler changes?  If not, then in future it would be better to submit multiple smaller pull requests, rather than monolithic ones.