SPARK-1242 Add aggregate to python rdd

Author: Holden Karau <holden@pigscanfly.ca>

Closes #139 from holdenk/add_aggregate_to_python_api and squashes the following commits:

0f39ae3 [Holden Karau] Merge in master
4879c75 [Holden Karau] CR feedback, fix issue with empty RDDs in aggregate
70b4724 [Holden Karau] Style fixes from code review
96b047b [Holden Karau] Add aggregate to python rdd