ENH: Add FindJsonCpp.cmake.

Since upstream has not committed a jsoncpp-config.cmake, this is helpful for
finding JsonCpp, especially when JsonCpp_DIR is passed a system version is
available.
Merge pull request #22 from thewtex/find-jsoncpp

ENH: Add FindJsonCpp.cmake.
BUG: Export the JsonCpp include directories in SlicerExecutionModelConfig.cmake.
BUG: include ParameterSerializerConfig.cmake in SlicerExecutionModelConfig.cmake.

To fix Slicer linking with SlicerExecutionModel when linking against it so
that the location of ParameterSerializerLib is transitively propagated by the
inclusion of ParameterSerializerConfig.cmake -> inclusion of
ParameterSerializerTargets.cmake.

Clobbered the older JsonCppConfig.cmake because upstream has not included this
yet.
Merge pull request #23 from thewtex/export-dep

Export dependencies (include directories and library locations) in SlicerExecutionModelConfig.cmake