Python interpreter group

Zeppelin already has pySpark interpreter in the Spark interpreter group.

It would be cool if we also have a plain python interpreter group, to integrate with available python ecosystem.

Initial implementation may look something like pySpark does and leverage py4j

%python

Things that need to be taken care of:

• Auto-completion
• third-party dependencies loading (pip, conda, virtualenv?)
• different interpreters: pypy, Jython, python2.7, python3