Add release helper script SourceTarball.bash

Port the Utilities/Maintenance/SourceTarball.bash from ITK 4.5.0 to VTK.
Update the URL and tarball names for VTK.

Change-Id: Id78cb69b781d646e0800322b50c2d2dba905124b
Port SourceTarball.bash to run on OS X

Create a compute_MD5 function to abstract the differences between
the Linux "md5sum" tool and the OS X "md5" tool.

Change-Id: I79fdb8347f99a17643a73179bb98dc8c671442b9
Teach SourceTarball.bash to split source and data tarballs

Teach Utilities/Maintenance/SourceTarball.bash to generate separate
archives for the source code, normal data, and large data.  Organize
the archive layout so that they can all be extracted on top of one
another.  Make the data archives extract into <prefix>/.ExternalData
to appear as if the content came with the source tree.

Add .ExternalData/README.rst to document the directory and indicate its
existence.  Change the BUILD_TESTING default to OFF when the file is not
present.  Teach SourceTarball.bash to leave this file out of the source
tarball and put it in the normal data tarball.  With this approach
users can download and build the source with or without the data and
never hit the network for test data by default.

Change-Id: I33007959c87ea60a9b11f49e5f7af351fc2ca9b7
Merge branch 'release-add-SourceTarball-script-rel' into release

Change-Id: I552252efc4ad5b1079cca3d826f71cd6db58919a