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

(cherry picked from commit e03bc379ee03fde0ee4fa578d3c39aae35c63f01)
Signed-off-by: Matei Zaharia <matei@databricks.com>
Call correct stop().

Oopsie in #504.

Author: Aaron Davidson <aaron@databricks.com>

Closes #527 from aarondav/stop and squashes the following commits:

8d1446a [Aaron Davidson] Call correct stop().
(cherry picked from commit 45ad7f0ca7be4a89c066b246b0fee0c0c2e7c759)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>