Added an environment variable so that developers can try new revisions
without editing the remote module description file.

For example, the HEAD of the remote module can be tested by executing
these git commands in the remote module checkout:

git pull

export RemoteGitTag=`git rev-parse HEAD`

Run cmake with
cmake -DModule_WikiExamples:BOOL=ON

For the first time, the remote module tag is an official release.

This brings in vtk-dicom 0.7.0

Recent changes to KWSys removed the vtksys_stl and vtksys_ios
compatibility layers

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