Make Obscura extend Backbone.Collection

Addresses: https://github.com/jmorrell/backbone.obscura/issues/17

Obscura proxies currently don't have any relation to Backbone Collections, but this can cause issues in some libraries that [check to see if the collection is an instanceof Backbone.Collection](https://github.com/gmac/backbone.epoxy/blob/master/backbone.epoxy.js#L33).

This changes Obscura proxies to inherit from `Backbone.Collection` and then overrides all of the collection methods. Those methods which modify the original collection are replaced with methods that throw an exception, which should help users avoid doing something weird.