[SPARK-1817] RDD.zip() should verify partition sizes for each partition

RDD.zip() will throw an exception if it finds partition sizes are not the same.

Author: Kan Zhang <kzhang@apache.org>

Closes #944 from kanzhang/SPARK-1817 and squashes the following commits:

c073848 [Kan Zhang] [SPARK-1817] Cosmetic updates
524c670 [Kan Zhang] [SPARK-1817] RDD.zip() should verify partition sizes for each partition