Adding SimpleITK Python threading, and progress/abort to SimpleFilters

Enabling SimpleITK Python threading. This unlock the Python GIL while
inside C++ code in ITK, while still having safe commands on events.

Updated SimpleFilters with support for SimpleITK 0.8rc1, and adding a
progress bar and abort button.

$ git log --oneline 3bc2fdc5ca5c72b3fbfc596213eecf89cec31b32..
c939ce0 Update to use namespace KernelEnum types for morphology
34e55c9 Verify the update-sitk update branch does not exist before json update
5cb03ef Merge branch 'sitk-upstream'
f46c414 SimpleITK 9f758175 Filter descriptions (reduced)
18db32e Adding Cancel button to abort filter processing.
757d0fe Adding ProgressBar and SimpleITK events