Remove DOM library hard dependency

Backbone only has a dependency on a DOM library if you intend to use
*Backbone.View*. If you don't include a DOM library, `Backbone.$` will be
`undefined`.

Since Backbone.localStorage doesn't have a hard dependency on
*jQuery.Deferred*, it should check for the existence of `Backbone.$`
before accessing one of its members otherwise this plugin raises
`Uncaught ReferenceError: $ is not defined`.

This makes it possible to use Backbone.localStorage with Backbone and
an alternative to *Backbone.View*.