bugfix: clustered annotations will no longer flicker when you move around on the map.
Merge commit '5d38acffdd2b5240fe91b4b489cd972128df66cc'

Conflicts:
	OCMapView/OCAnnotation.m
bugfix: grid cluster computing works again

- I inadvertedly broke it recently...
Merge pull request #31 from michaelpeternell/master

some bugfixes...