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.

(cherry picked from commit 5c7faecd75ea59454ad3209390ac078e6cf6e4a6)
Signed-off-by: Reynold Xin <rxin@apache.org>