Added scripts/graceful-redis-brain.coffee.

This module uses the redis brain if REDISTOGO_URL is explicitly defined,
otherwise it does nothing. This allows developers to hack on hubot
without having to install redis, while allowing redis to be easily
enabled if needed.

If a developer wants to use their local instance of redis, they can
set REDISTOGO_URL to redis://localhost:6379. (Previously, they needed
to do nothing, but then hubot would have complained if the local
redis server couldn't be contacted.)
Merge pull request #12 from toolness/master

Only enable the redis brain if REDISTOGO_URL is defined