MAHOUT-1580 Optimize getNumNonZeroElements() closes apache/mahout#17
MAHOUT-1573: More explicit parallelism adjustments in math-scala DRM apis; elements of automatic parallelism management

This closes apache/mahout#13.

Squashed commit of the following:

commit de03a6aa8361424ee8fb776f995fbe1b811e0ccd
Author: Dmitriy Lyubimov <dlyubimov@apache.org>
Date:   Wed Jun 18 14:22:46 2014 -0700

    doc

commit f399f7a4dc61905ef05d9944dbd2e5a4c31a654b
Merge: b02cf18 45d8803
Author: Dmitriy Lyubimov <dlyubimov@apache.org>
Date:   Wed Jun 18 14:18:35 2014 -0700

    Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/mahout into MAHOUT-1573

commit b02cf18dac608e2f969a845efd7ee35a3a5bd0e0
Author: Dmitriy Lyubimov <dlyubimov@apache.org>
Date:   Wed Jun 18 14:18:03 2014 -0700

    switching to par(...) api

commit 06bb4bcdedb3c5aac50a44ddec3957f1ed12d808
Author: Dmitriy Lyubimov <dlyubimov@apache.org>
Date:   Fri Jun 6 23:05:12 2014 -0700

    pom fixes to fix tests (spark hadoop dependencies must precede that of mahout-math's for tests to run correctly)

commit 1e6e3f87ffab5897569fea94c643da2bdbb59e33
Author: Dmitriy Lyubimov <dlyubimov@apache.org>
Date:   Fri Jun 6 22:15:33 2014 -0700

    remove any special handling of Par in rewrites

commit 2f3b4f5bac901ec152453473d4982cb9b6e5d651
Author: Dmitriy Lyubimov <dlyubimov@apache.org>
Date:   Fri Jun 6 16:33:27 2014 -0700

    + auto_|| operator

commit 2733002f4b5db3d5114a440b03967d954a3738e9
Author: Dmitriy Lyubimov <dlyubimov@apache.org>
Date:   Fri Jun 6 15:56:26 2014 -0700

    explicit parallelism adjustment levers exact_|| and min_||

commit cf7f18b4af4ad043d2bdcefeeda15031fa018543
Author: Dmitriy Lyubimov <dlyubimov@apache.org>
Date:   Fri Jun 6 13:21:19 2014 -0700

    docs

commit 2f785109b9a52e748626ba46f5bc0a35ffc98e2c
Author: Dmitriy Lyubimov <dlyubimov@apache.org>
Date:   Fri Jun 6 13:02:11 2014 -0700

    Refactoring drmFromHDFS()