Remove underscore dependency.

Hey there.

I develop Backbone fork called [Exoskeleton](http://exosjs.com) — basically a faster and leaner Backbone with optional dependencies. Underscore and jQuery still can be used, but it's up to user.

So, I wanted to remove these deps from my application, but bb.localStorage uses underscore.

I have removed all references which make your plugin compatible with exoskeleton and useful for people that don't want to include deps!