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 (?)
(NOJIRA) shell -- context cleanup (Anand Avati via dlyubimov)

This closes apache/mahout#15

Squashed commit of the following:

commit 0407577bff365028ceef9798d5835f12a81bc941
Author: Anand Avati <avati@redhat.com>
Date:   Mon Jun 9 22:05:14 2014 -0700

    MAHOUT-1529: spark-shell - cleanup sdc

    Renaming @sc to @sdc has left open the shutdown part. Spark
    depends on 'implicit sc' to be set by the shell invoker.

      mahout> Stopping spark context.
      <console>:29: error: not found: value sc
                    sc.stop()
                    ^

    So override sparkCleanUp() and call @sdc.stop()

    Signed-off-by: Anand Avati <avati@redhat.com>