PEP8
Use AuthorSerializer in BlogPostSerializer.

Conflicts:

	apps/blogs/serializers.py
Disable django.contrib.sites tests in Jenkins.
Add MediaWallPostPhoto.
Install Pillow in Travis env.
Merge pull request #167 from onepercentclub/mediawallpostphoto

Add MediaWallPostPhoto.
Merge branch 'master' into wallpost-reactions-crud