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

Closes #6183 from sun-rui/SPARK-7227 and squashes the following commits:

dd6f5b3 [Sun Rui] Rename readEnv() back to readMap(). Add alias na.omit() for dropna().
41cf725 [Sun Rui] [SPARK-7227][SPARKR] Support fillna / dropna in R DataFrame.