[SPARK-9324] [SPARK-9322] [SPARK-9321] [SPARKR] Some aliases for R-like functions in DataFrames

Adds following aliases:
* unique (distinct)
* rbind (unionAll): accepts many DataFrames
* nrow (count)
* ncol
* dim
* names (columns): along with the replacement function to change names

Author: Hossein <hossein@databricks.com>

Closes #7764 from falaki/sparkR-alias and squashes the following commits:

56016f5 [Hossein] Updated R documentation
5e4a4d0 [Hossein] Removed extra code
f51cbef [Hossein] Merge branch 'master' into sparkR-alias
c1b88bd [Hossein] Moved setGeneric and other comments applied
d9307f8 [Hossein] Added tests
b5aa988 [Hossein] Added dim, ncol, nrow, names, rbind, and unique functions to DataFrames