Add ol.source.Source#dispatchChangeEvent
Propagate change events from sources via layers
Merge remote-tracking branch 'ol3/master' into pin

Conflicts:
	src/ol/style.exports
Fixed text pin symbolizer
Merge remote-tracking branch 'origin/pin' into private-master
Merge pull request #918 from twpayne/source-change-events

Source change events
Merge remote-tracking branch 'ol3/master' into private-master