This adds Python API for those DataFrame functions that is introduced in 1.5.

There is issue with serialize byte_array in Python 3, so some of functions (for BinaryType) does not have tests.

cc rxin

Author: Davies Liu <davies@databricks.com>

Closes #7922 from davies/python_functions and squashes the following commits:

8ad942f [Davies Liu] fix test
5fb6ec3 [Davies Liu] fix bugs
3495ed3 [Davies Liu] fix issues
ea5f7bb [Davies Liu] Add python API for DataFrame functions