Add redis session support

Implementation taken from here - http://flask.pocoo.org/snippets/75/
I wasnt sure if this was the best filename to use, but the next best thing I could think of was having `session/redis.py` and that didnt allow me to import redis (I assume because it was a module called redis trying to import itself)

If there is a workaround to that and that name seems to be better, i'm happy to change it.