Enhancements to RPC, and make server-set meta available.

1. allow RPC calls against the collection (ie, matching the following import route in a rails api):

    resources :widget do
      collection do
        post 'import' => :import
      end
    end

2. Make meta property set on the server always available. The implementation is somewhat ugly, but the result is that whatever you do that hits the server, the meta info will be available in the then of the promise. In the case of a flush, each of the models in the result set passed to the `then` of the flush will have the `meta` that was set by its (last) operation. In the case of a query, the `meta` will be set on the model array. It is set as bare javascript property, not as an ember property. Should I change this?

3. Allow choosing an HTTP method for RPC (by setting _method in params)