Handle client side association collection removals

Association collections with items removed will now also be removed on the server side on save.

Fixes #15

* Supports any method that modifies an association array through `method_missing`
* Using `ActiveRecord::Associations::CollectionProxy#delete` for item removal.
* trying to remove items from has_many through and habtm associations do not work, and we log an informative error.
* Supports Rails 4.0 as well as 4.1 (Rails 4.1 converted habtm associations to work as has_many through)

Same behavior as before, but not sure if we want to update the association with the `Array` or keep it as a `ResourceProxy` 
Not sure if updating to `Array` is an oversight or essential.