Mahout 1464

MAHOUT-1464 looks ready to me but can't push it yet.

My build is broken from an unrelated mr-legacy test so I'll wait to push it until my local build passes but wanted to get this out for review if anyone cares.

I took out the epinions and movielens examples, will add them back in with the CLI driver maybe. 
There are few doubts there. 
(1) DSL follows R. Which R function this mimics? If none, can we get away with doing it on java side? ( i guess we can't get around it if you want to do it for DRMs). 
(2) the name IMO is misleading given description. nonZeroCounts()?
(3) The implementation (both in-core and distributed, BTW) seem to contradict the description. It looks like implementation actually counts number of _non-negative_ elements rather than what description portrays.
(4) I would like Sebastian's feedback on this too, since he is both the primary co-occurrence author and understands R-like semantics idea very deeply.  If there's a better semantics (like I suspect it should be), he should know that.