Updated Java, Scala, Python, and R.

Author: Reynold Xin <rxin@databricks.com>
Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #5996 from rxin/groupby-retain and squashes the following commits:

aac7119 [Reynold Xin] Merge branch 'groupby-retain' of github.com:rxin/spark into groupby-retain
f6858f6 [Reynold Xin] Merge branch 'master' into groupby-retain
5f923c0 [Reynold Xin] Merge pull request #15 from shivaram/sparkr-groupby-retrain
c1de670 [Shivaram Venkataraman] Revert workaround in SparkR to retain grouped cols Based on reverting code added in commit https://github.com/amplab-extras/spark/commit/9a6be746efc9fafad88122fa2267862ef87aa0e1
b8b87e1 [Reynold Xin] Fixed DataFrameJoinSuite.
d910141 [Reynold Xin] Updated rest of the files
1e6e666 [Reynold Xin] [SPARK-7462] By default retain group by columns in aggregate