Feature/Config Redis Brain AutoSave

I've added configurable auto saving to the redis brain script. 
It uses:
HUBOT_BRAIN_AUTOSAVE=[true|false]
HUBOT_BRAIN_AUTOSAVE_INTERVAL=\<seconds\>

Let me know it there is anything out of place with my changes.

Also, if this type of feature is not needed/wanted; I do apologize for the bother.