[SPARK-9318] [SPARK-9320] [SPARKR] Aliases for merge and summary functions on DataFrames

This PR adds synonyms for ```merge``` and ```summary``` in SparkR DataFrame API.

cc shivaram

Author: Hossein <hossein@databricks.com>

Closes #7806 from falaki/SPARK-9320 and squashes the following commits:

72600f7 [Hossein] Updated docs
92a6e75 [Hossein] Fixed merge generic signature issue
4c2b051 [Hossein] Fixing naming with mllib summary
0f3a64c [Hossein] Added ... to generic for merge
30fbaf8 [Hossein] Merged master
ae1a4cf [Hossein] Merge branch 'master' into SPARK-9320
e8eb86f [Hossein] Add a generic for merge
fc01f2d [Hossein] Added unit test
8d92012 [Hossein] Added merge as an alias for join
5b8bedc [Hossein] Added unit test
632693d [Hossein] Added summary as an alias for describe for DataFrame