SPARK-1862: Support for MapR in the Maven build.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #803 from pwendell/mapr-support and squashes the following commits:

8df60e4 [Patrick Wendell] SPARK-1862: Support for MapR in the Maven build.
(cherry picked from commit 17702e280c4b0b030870962fcb3d50c3085ae862)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>