Added PEP8 style configuration file.

This sets the max line length to 100 as a PEP8 exception.

Author: Reynold Xin <rxin@apache.org>

Closes #872 from rxin/pep8 and squashes the following commits:

2f26029 [Reynold Xin] Added PEP8 style configuration file.

tox.ini

+[pep8]
+max-line-length=100