SPARK-1162 Added top in python.

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

Closes #93 from ScrapCodes/SPARK-1162/pyspark-top-takeOrdered and squashes the following commits:

ece1fa4 [Prashant Sharma] Added top in python.

(cherry picked from commit b8afe3052086547879ebf28d6e36207e0d370710)
Signed-off-by: Matei Zaharia <matei@databricks.com>