(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>