Add CMake code to fetch testing data.

The ExternalData.cmake module is used to fetch testing data.

All the content links (.md5 files) found in Testing/Data/
are fetched when the SimpleITKData target is built.

Change-Id: I4b7cf9545e7aa3bac9a129aabb3c0d54f90f40dc
Remove the Testing/Data submodule.

Replacement content links to be added in the next patch.

Change-Id: I51645a4e32f659e2f6b85e9be12d435996fa0401
Add testing data content links from submodule.

Also change the TEST_HARNESS_DATA_DIRECTORY to point to the
ExternalData directory in the binary tree and ImageCompare
to point to the binary tree.

Change-Id: I8a5f8bae661a13177ad5418adc18820869fd6240
ENH: Use CMake ExternalData.

Improves maintainability.

If BUILD_TESTING is not CMake 2.8.11 is required (initial distribution
of ExternalData.cmake).

Change-Id: I202f33bdeb2eecea67af676806ff6417db30e217
Merge topic 'SIMPLEITK-355_CMakeForMidasAndMD5' into next

cc4d6d9 ENH: Use CMake ExternalData.
11fab91 Add testing data content links from submodule.
48fe398 Remove the Testing/Data submodule.
6dfc0c1 Add CMake code to fetch testing data.