Add preliminary support for SparkR

Add basic connection, ddo/ddf, and MapReduce methods to tie in to
SparkR backend.  This is extremely experimental at the moment.
kvSparkData objects can only be initialized with in-memory R objects.
A simple MapReduce job can be run.  More experimentation to come...
Fix Spark MapReduce to properly broadcast and use 'params' object
Add Spark unit tests
Fix multiple extraction and add relevant tests

Multiple extraction of memory and local disk ddo/ddf objects by
key and by key hash did not necessarily return k/v pairs in the
order specified.  Now it does.
Merge branch 'SparkR' into dev