Redis session state

Depends on #49 
Resolves issue #48 

Initially I looked into connect-pg for storing session state in postgres, but there were necessarily quite a few dependencies and setup steps to create the schema objects required for persisting sessions in the database. I hesitated on that path because of the extra setup complexity. 

I tried out connect-redis and was very pleased with how simple it is to integrate. Seems to work great -- restarting node does not kick the users off.

Installed redis on the server in case we want to merge this in: 
````
apt-get install redis-server
````