Performance enhancements & UI

These two commits address one performance bottlenecks and general UI feeling.

In ab202a5 the invalidate method is called once the data loading thread is finished. So far, it was called, while the Overlay was being populated and so often not all items were draw initially. This was moved into an AsyncTask,

782816c limits the onDrawItem() calls to items in the Overlay to PlakatOverlayItems that are close to the boundingbox of the MapView, thus movement on the map is much smoother.

Hope you can merge them, thanks for the nice app.
Dahie