[SPARK-7482] [SPARKR] Rename some DataFrame API methods in SparkR to match their counterparts in Scala.

Author: Sun Rui <rui.sun@intel.com>

Closes #6007 from sun-rui/SPARK-7482 and squashes the following commits:

5c5cf5e [Sun Rui] Implement alias loadDF() as a new function.
3a30c10 [Sun Rui] Rename load()/save() to read.df()/write.df(). Also add loadDF()/saveDF() as aliases.
9f569d6 [Sun Rui] [SPARK-7482][SparkR] Rename some DataFrame API methods in SparkR to match their counterparts in Scala.