Enabling backend definition at instantiation time

Right now you must extend `Backbone.Collection` to give it a backend. In many cases only a master instance that syncs everything is needed, so I find it easier to directly instantiate it and pass the backend with the options if no additional methods or attributes are required.
This is how the native `Backbone.Collection.url` works, which I think is the natural predecessor of backends.
Sorry about the ugly whitespace diff, my editor automatically trims it.