MAHOUT-1606 - Add rowSums, rowMeans and diagonal extraction operations to distributed matrices
this closes apache/mahout#43

Squashed commit of the following:

commit cd819a5df79e43d558d450ef75c76ae47fcd525b
Merge: fd54249 4bac5c2
Author: Dmitriy Lyubimov <dlyubimov@apache.org>
Date:   Fri Aug 15 12:07:21 2014 -0700

    Merge branch 'master' into rowmeans

commit fd54249c20a5c89c96060b6d96e49ada579d8144
Author: Dmitriy Lyubimov <dlyubimov@apache.org>
Date:   Tue Aug 12 16:15:32 2014 -0700

    rowSums, rowMens and diagonal extraction for distributed matrices