Add "destroying" and "destroyed" signals to BaseResourceModel.
Review Request #4115 — Created May 8, 2013 and submitted
Add "destroying" and "destroyed" signals to BaseResourceModel. "destroying" is triggered as soon as the destroy() call is called. "destroyed" is triggered after a successful destroy. Note that Backbone.js provides a "destroy" signal, but this doesn't follow the naming conventions we're otherwise using.
Unit tests pass. Used this in another change.