Jira: https://issues.apache.org/jira/browse/SPARK-8727

Author: Tarek Auel <tarek.auel@gmail.com>
Author: Tarek Auel <tarek.auel@googlemail.com>

Closes #7114 from tarekauel/missing-python and squashes the following commits:

ef4c61b [Tarek Auel] [SPARK-8727] revert dataframe change
4029d4d [Tarek Auel] removed dataframe pi and e unit test
66f0d2b [Tarek Auel] removed pi and e from python api and dataframe api; added _to_java_column(col) for strlen
4d07318 [Tarek Auel] fixed python unit test
45f2bee [Tarek Auel] fixed result of pi and e
c39f47b [Tarek Auel] add python api
bd50a3a [Tarek Auel] add missing python functions