Add ol.Observable#getRevision
Add ol.Observable#dispatchChangeEvent
Use inherited dispatchChangeEvent and getRevision in ol.source.Source
Use inherited dispatchChangeEvent and getRevision in ol.Feature
Use inherited dispatchChangeEvent and getRevision in ol.geom.Geometry
Use inherited dispatchChangeEvent in ol.layer.Base
Merge pull request #1563 from twpayne/revision-in-observable

Move dispatchChangeEvent and getRevision into ol.Observable