adds dist and cluster
adds CLUSTERING_THRESHOLD calculation
adds screenX and screenY temporarily
adds grouping members to overlayitem
adds getGroupSet
fixes threshold's proportion of the screen, adds currentGroup
Cleanup and tests

* Add tests for MapTile
* Add tests for TileLayer
This is the beginning of a rewrite of how we do pinch and zoom

gestures.

The old-fashioned MultiTouchController is made obsolete by built-in
Android APIs, so we use them. And we support panning while pinching
and loading tiles while doing anything.
fixes getting center by using angles and not means (duh)
adds getThreshold
adds temp special overlay for cluster markers
chage location of log point so that it actually asserts that the marker has been loaded
fixes initialization of cluster overlay
changes default pivoting point to center-center
generates marker with cluster
adds method to remove a marker from the default itemized overlay
not drawing if item is currently inside a cluster
adds invalidate to hotspot setting
adds property to check if clustering is enabled for this overlay
sets property on cluster match