BUG: FindJsonCpp wrong library identifier.

libjson.so is provided by the json-c project, not JsonCpp.

Also fix find_package_handle_standard_args variables.

Change-Id: Ia335dfc73f9651ef5ced017ddb4d151b7c7687f5
Merge branch 'JsonCppLibName'

Change-Id: I513b89daab61e5a44dbbf97de5c17e9ec394a41b
Merge branch 'master' of git://github.com/TubeTK/TubeTK into Slicer4221Dependencies