COMP: Enable superbuild for Slicer4 extension.

JsonCpp and ParameterSerializer will be built in superbuild directory while Slicer provides everything else.

Change-Id: Id4be148c89c864cb75dcb2ff6451ad993e0a58ce
Merge pull request #166 from chrismullins/enable-superbuild-slicer-extension

COMP: Enable superbuild for Slicer4 extension.