Fix PEP8 violations in Python mllib.

Author: Reynold Xin <rxin@apache.org>

Closes #871 from rxin/mllib-pep8 and squashes the following commits:

848416f [Reynold Xin] Fixed a typo in the previous cleanup (c -> sc).
a8db4cd [Reynold Xin] Fix PEP8 violations in Python mllib.

(cherry picked from commit d33d3c61ae9e4551aed0217e525a109e678298f2)
Signed-off-by: Reynold Xin <rxin@apache.org>