Add change event to Geolocation

This PR makes Geolocation instances dispatch `change` events when the geolocation changes.

Currently the [example in the API doc](http://ol3js.org/en/master/apidoc/ol.Geolocation.html) suggests listening to `change:position` to get notified when the geolocation changes. But speed and  accuracy_geometry are not yet up-to-date when this event occurs. This PR fixes this problem by introducing a `change` event dispached when all the geolocation properties have been updated.

Please review.