ENH: Remote module capability

A preliminary implementation of Remote modules. This approach follows
the ITK implementation.

Three remote modules are included for testing
 David Doria's Poissoin Reconstruction, David Gobbi's vtkDICOM and Slicer's vtkAddon.

To configure vtk with these remote modules:
cmake -DvtkAddon:BOOL=ON -DModule_PoissonReconstruction:BOOL=ON -DModule_vtkDICOM:BOOL=ON vtk_source_dir
make