performance improvments moved to extension/markerclusterer.js
issue #502
save progress
severe extension / override of MarkerClusterer, underlying array is now
a PropMap/ Object
idKey is required for Marker now as well for MarkerManager
Merge branch 'issue_502_MarkerClusterer_perf_impovements' into develop

Conflicts:
	dist/angular-google-maps.js
	dist/angular-google-maps.min.js