Fix createDataFrame() from pandas DataFrame (not tested by jenkins, depends on SPARK-5693).

It also support to create DataFrame from plain tuple/list without column names, `_1`, `_2` will be used as column names.

Author: Davies Liu <davies@databricks.com>

Closes #4679 from davies/pandas and squashes the following commits:

c0cbe0b [Davies Liu] fix tests
8466d1d [Davies Liu] fix create DataFrame from pandas