John/geojson feature

This branch adds a vtkGeoJSONMapFeature class, for importing GeoJSON data into vtkMap. A vtkPolyData is first constructed for an input GeoJSON object -- either feature or feature collection. That vtkPolyData object can then be added to any feature layer.

Note: this branch adds a new build dependency on the vtkDataSetReaders repository at https://github.com/OpenGeoscience/vtkDataSetReaders.

Also to Aashish: I prefer that this branch get merged before the multithreaded branch.