ENH: Add GENERATE_TCLAP_ASSIGNMENT_IFSET generation.

The GENERATE_TCLAP macro is split into GENERATE_TCLAP_PARSE
and GENERATE_TCLAP_ASSIGNMENT.  Another macro is also created
that assigns the C++ variables only if they have been set on the command line.

Currently, GENERATE_TCLAP_ASSIGNMENT is not used by PARSE_ARGS, but it is used
by TubeTK to overwrite parameters that have not been set.
ENH: Add ParameterSerializer support.

Add support for the parameter serialization with
ParameterSerializer (https://github.com/TubeTK/TubeTK-ParameterSerializer).
A module will automatically get new flags to serialize and deserialize
the parameters of the module to/from a JSON file.

Support is enabled by setting SlicerExecutionModel_USE_SERIALIZER to ON,
which will require the specification of the ParameterSerializer build tree,
ParameterSerializer_DIR.
Merge pull request #20 from thewtex/parameter-serializer

Parameter serializer support.