Backbone learns 'sync:fetch', 'sync:save', and 'sync:destroy' triggers

On occasion, one would like to get notifications for save w/o getting triggers for fetch or destroy. 

I've gone ahead and created this change that adds all three types of sync changes to model/collection (where applicable). It should cover other people's use cases and won't break anyone's existing functionality (Unless of course they already use sync:fetch, sync:save, or sync:destroy as custom triggers on their models..).