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.