… listeting to Firebase if they are no longer needed.
add missing urlError method save the original Backbone.sync reference as sync_AJAX updated documentation
Previously, the lib did not set the model.id attribute when populating a collection from firebase. This meant that the models could not be identified when `child_changed` or `child_removed` events came down the pipe. This commit introduces a configurable `idAttribute`, which is populated with the firebase reference name. Implementations then need to set the `idAttribute` of their model class to use this as the source for the `model.id` value. I've updated the readme accordingly but it might be worth providing a `FirebaseModel` class that does this automatically, or generally considering the best implementation. In addition, I've patched up the update and delete event handlers to get the existing model by id (rather than trying to `collection.get(pushed_mode.val())`.