Initial ideas for couchbase-backed storage API.

@zaach here is an initial attempt at a lightweight storage abstraction on top of couchbase, based on how it's organised in gombot but without the gombot-api-specific layer on top.

It's not ready for merge but keen to get some feedback - it's the first node-js code I've written in a long while, so please be absolutely brutal w.r.t. layout, conventions, nitpicks and whatever other problems you might see!