Fix a couple multithreading issues

* Start background thread *after* initializing internal data
* Assign layer to map tiles in foreground thread
Install vtkMapTileSpecInternal.h as it is required by vtkOsmLayer.h
Merge pull request #69 from OpenGeoscience/properly_install_vtkMapTileSpecInternal

Install vtkMapTileSpecInternal.h as it is required by vtkOsmLayer.h
Merge pull request #68 from OpenGeoscience/john/multithread-fixes

Fix a couple multithreading issues
Merge branch 'master' into databrowser