Add ajaxPools to Model/Collection

This fixes a small memory issue (and potential state management issue) in Chaplin: Although Chaplin disposes of Models/Collections on a Controller with every URL change, those models/collections might still have running Ajax requests (and can cause havoc when they return to callbacks on disposed Controllers/Views). This cleans all that up, exposes an "api" method on Model, and also adds a very basic Error controller.