Add ExternalData infrastructure

Port the ExternalData configuration from ITK 'master' as of 2013-05-06.

Add a CMake/vtkExternalData.cmake module to include and configure the
ExternalData module for VTK.  List MIDAS and vtk.org URLs.

Include vtkExternalData from the top-level CMakeLists.txt file and from
Testing/External/CMakeLists.txt so it works for both the main build and
when building tests externally.

Teach the VTK pre-commit hook to move staged .ExternalData_MD5_* files
left by the ExternalData module into the store at the top of the source
tree as .ExternalData/MD5/* and import them into Git as refs/data/MD5/*.
Teach the VTK git-gerrit-push alias to push content from refs/data/MD5/*
when commits referencing it are pushed to Gerrit.

Add a .gitignore to tell Git to ignore .ExternalData* names.

Change-Id: I8c0a91cb3ce350450e378e4f16a23c62e7f2de6f