[SPARK-2199] [mllib] topic modeling

I have implemented Probabilistic Latent Semantic Analysis (PLSA) and Robust PLSA with support of additive regularization (that actually means that I've implemented Latent Dirichlet Allocation too).
Use commons-math3 if you please. We already spent time pruning other math libraries and I'd rather not bring back colt. (Versions go in the parent POM, as an aside)