Add ability to override storage engine for sessions.

[dcbartlett]

To override the default storage engine for your application's session store. Just add the following to config/application.js

session: {
    adapter: 'mongo',
},

This will override the default that is setup in the Sails.JS core, allowing you to use mongodb for your session stores.

[mikermcneil]

This is working for 'redis' in master--

Technically, any Sails adapter that supports CRUD could be used as a session store this way if we put together a generic connect-sails-adapter module to do the mapping. I think that might be the best way to do it-- different projects are going to necessitate all different kinds of session store support.

[mikermcneil]

Closing this for now since we have Redis support- next step is an agnostic connect-* adapter that works w/ any waterline adapter.

Also see #405 for a hypothetical method of solving this