ENH: SourceTarball.bash: also fetch from Midas.

Many testing data files are on midas3.kitware.com and not on
www.itk.org/files/.

Change-Id: Iac8f8dd01a4882085f4e13c34ba7403922f67e7b
ENH: Add .ExternalData to .gitignore.

Holds ExternalData not to be committed.

Change-Id: Ib1f7d44f2a8b3c7998db2a89e3bbf837fa939548
Merge topic 'SIMPLEITK-416_SourceTarballData'

200a30d ENH: Add .ExternalData to .gitignore.
82eca71 ENH: SourceTarball.bash: also fetch from Midas.