SPARK-1322, top in pyspark should sort result in descending order.

Author: Prashant Sharma <prashant.s@imaginea.com>

Closes #235 from ScrapCodes/SPARK-1322/top-rev-sort and squashes the following commits:

f316266 [Prashant Sharma] Minor change in comment.
58e58c6 [Prashant Sharma] SPARK-1322, top in pyspark should sort result in descending order.