Added unsnappedChanges method to resource with snapshot mixin

I'm planning on implementing the snapshot mixin, and I thought it'd be convenient to have a way to detect if the resource had any changes compared to the latest snapshot.

Since `angular.equals` ignores $-prefixed keys, this would not detect changes made to, say, `$key` in the specs, but it does detect a `name` change from "The Winds of Winter" to "Harry Potter".