Issue3 mbtiles

I transferred now all functionality to the Overlay class.

You might not want to pull my open-streets-dc.mbtiles file (as your files are way better suited I think and it would take up useless space in the repo forever).
I would then (in case you pull this) nuke my fork and refork again, to get rid of my mbtiles file as well and then include your old mbtiles files and update the viewcontroller accordingly.
Does this make sense to you or am I talking in riddles?

Interestingly I always thought that Apple removes an overlay when going over its maximumZ but it is not that way (anymore?). Apple now does its own interpolating, but not very predictable (it will only interpolate what is still in cache it seems)