John/vtk multi threaded osm layer

Aashish, I *think* I've got the multithreaded logic working, and wanted to see if you would give it a try.

* There is a new vtkMultiThreadedOsmLayer class that does concurrent http requests in the background and updates the display asynchronously (via polling).
* This branch includes a new test program TestMultiThreadedOsmLayer to run the new capability.



Just for the docuementation ponit of view. I think we should look into deferred objects so that Map just check at a single location and not asking each layer if they are async or not. We discussed this already but just wanted to write it here so that I don't forget about it.