MAHOUT-1572: blockify() to detect (naively) the data sparsity in the loaded data

This closes apache/mahout#10

Squashed commit of the following:

commit cc93dedcad0b6ff0365f8e15dba280221a0a64f0
Author: Dmitriy Lyubimov <dlyubimov@apache.org>
Date:   Tue Jun 10 11:27:38 2014 -0700

    + tests

commit 07180efbfa4472f7d13a2afe1f8f37e18edfe08e
Merge: 162c5ca 907781b
Author: Dmitriy Lyubimov <dlyubimov@apache.org>
Date:   Tue Jun 10 11:04:51 2014 -0700

    Merge branch 'master' into MAHOUT-1572

commit 162c5ca36e00af91a9599075332c577d9b1a13c4
Author: Dmitriy Lyubimov <dlyubimov@apache.org>
Date:   Wed Jun 4 15:10:11 2014 -0700

    initial fix (?)