Event antiflood

Synchronous version of #22 , implementation of https://github.com/ractivejs/ractive-adaptors-backbone/pull/22#issuecomment-57810303 idea.

Decorate (and undecorate) `collection.sync` method to intercept update and receive pre`sync` event (https://github.com/jashkenas/backbone/blob/1.1.2/backbone.js#L863 https://github.com/paulmillr/exoskeleton/blob/0.7.0/exoskeleton.js#L1090)

Lock all update events during ajax-driven collection update and then trigger custom update event for all listening `BackboneCollectionWrapper`s.

Old synchronous tests approve this patch.