Added checks when zooming maps and fixed an issue where  the map zoomed to level 19 automatically on double tap.

The minimum zoom level is now 0 by default, the maximum 22 by default.
Setters of MapZoom, MapMaxZoomLevel and MapMinZoomLevel now automatically correct any out of bounds zoom levels. This makes the NormalizeZoom method redundant.
MAX_ZOOM_LEVEL should not be static.
Fixed wrong check in MapZoom setter.
Merge pull request #134 from txsmith/master

Fix double-tap zoom issue for iOS