[SPARK-7060][SQL] Add alias function to python dataframe

This pr tries to provide a way to let python users workaround https://issues.apache.org/jira/browse/SPARK-6231.

Author: Yin Huai <yhuai@databricks.com>

Closes #5634 from yhuai/pythonDFAlias and squashes the following commits:

8465acd [Yin Huai] Add an alias to a Python DF.