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
Andrew could you update the HACKING.txt file with the preferred coding standards so we can follow them? The Scala style guide referenced at the bottom of that file recommends interleaving newlines between methods, so you should mention where you want FACTORIE code to deviate from that style.