Adds ClusterItem as subclass of Marker
adds cluster circle as an image
adds fallback marker
removes invalidate right after addItem, as remote marker won't be ready by then
sets density of marker bitmaps on load
sets offline marker if no Maki icon is specified
refactors to use ClusterItem instead of Marker
not clustering if there's no more than one item in the group
sets cluster special marker up
draws the number of clustered items if the item being drawn is a cluster marker