Unique event type value for this target

The goog.events.EventType.CHANGE type is already dispatched because layers are ol.Object instances.  To allow people to register for vector layer specific events without also having listeners get called with more general ol.Object change events, we need to give the types unique values.
Extend goog.events.Event for custom vector layer events

Vector events include features and extents.  In the case of feature change events, the extents array will include old and new extents for features with modified geometries.w
Merge pull request #1092 from tschaub/vector-event

Extend goog.events.Event for custom vector layer events.