MAHOUT-1365 Implicit feedback ALS-WR

in-core and distributed versions. Per document. 

What possibly is lacking at this point
(1) an option to do non-WR version (just simplle implicit feedback). 
(2) MSE computations and convergence test.
(3) closer tweaks for in-core math performance. One of the things side-by-side rbind of U,A and V,A' to mimic vector tuples without getting hands dirty with actual tuple-filled RDDs. Can it work reasonably well.