Added core functionality for thumbnails.
Refined our sorl thumbnail config a bit.

If you're using the dev settings you can simply use your existing DB
as your sorl thumbnail key-value store. If you're in prod, you'll need
to supply a Redis service.

Note! sorl-thumbnail is a new app. You should run syncdb.

Fixes #510
Merge branch 'develop' of github.com:harvard-lil/perma into develop
Bug fix: I played a bit with thumbnailing the archive image and
left some experimental code in. Rolling back.
Fix Indiana logos.
Merge pull request #694 from jcushman/indiana

Fix Indiana logos.