MapView should expose zoom property.

Currently, you can't access the Zoom property of MapView. The set/get zoom function only returns the preferred zoom, not the actual zoom.

MapView has access to the BaseMap (which contains this much-needed zoom property). However, BaseMap is not exposed by the module. The easiest solution is to add a function, zoomProperty(), that forwards the call to BaseMap.

relates to #87 and #69 
