Limit the MapView with a bounding box of geo coordinates

Hi there,

For another enhancement, here is my stab at issue #55. Works on both Android and iOS. By setting the MapBoundingBox property on a Map, it will stay within the boxs' bounds when panning and zooming. 

It works nice and smooth on iOS. I have not tested this on an actual Android device, but it worked fine in the emulator.

One possible improvement would be to automatically fit the view to the box if the view is to large to be contained within the box.

Kind regards,
Thomas Smith