Fixed onChanged for events

Based on documentation (http://fullcalendar.io/docs/event_data/updateEvent/)

> event must be the original Event Object for an event, not merely a reconstructed object. The original Event Object can obtained by callbacks such as eventClick, or by the clientEvents method.

I changed ```eventsWatcher.onChanged``` to work with original Event Object obtained by ```clientEvents``` method based on ```_id```

This should fix #186