Support order by and sort by on Spark [Spark Branch]

Currently Hive depends completely on MapReduce's sorting as part of shuffling to achieve order by (global sort, one reducer) and sort by (local sort).
Spark has a sort by transformation in different variations that can used to support Hive's order by and sort by. However, we still need to evaluate weather Spark's sortBy can achieve the same functionality inherited from MapReduce's shuffle sort.

Currently Hive on Spark should be able to run simple sort by or order by, by changing the currently partitionBy to sortby. This is the way to verify theories. Complete solution will not be available until we have complete SparkPlanGenerator.

There is also a question of how we determine that there is order by or sort by by just looking at the operator tree, from which Spark task is created. This is the responsibility of SparkPlanGenerator, but we need to have an idea.