BUG: FindJsonCpp wrong library identifier.

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

Also fix find_package_handle_standard_args variables.
Merge pull request #5 from thewtex/JsonCppLib

BUG: FindJsonCpp wrong library identifier.
COMP: Update FindJsonCpp.cmake
BUG: Fix Gaussion -> Gaussian typo.
ENH: Add GradientDescentOptimizerSerializer.

Also supporting itkObjectSerializer.h

ITK_EXPORT removed.
COMP: Use manual listing for include_directories.
Merge pull request #6 from thewtex/GradientDescentOptimizer

Gradient descent optimizer
ENH: Update Travis CI configuration to use ITK snapshot
Merge pull request #7 from jamiesnape/UpdateTravisCI

ENH: Update Travis CI configuration to use ITK snapshot
ENH: Use references in Get methods for *ArrayValue's.

Avoid unnecessary copies.
BUG: Wrong types for FloatArrayValue and DoubleArrayValue.

in JsonCppArchiver.
ENH: Add OptimizerSerializer.
Merge pull request #8 from thewtex/GradientDescentBase

Gradient descent base
ENH: Use JsonCpp_DIR if provided.
Merge branch 'JsonCppConfig'
BUG: Use JsonCppConfig.cmake only if the file is present.

Co-authored-by: Jamie Snape <jamie.snape@kitware.com>
BUG: Scale cannot be negative in optimizer test.
Merge pull request #9 from thewtex/JsonCpp_DIRDefined

BUG: Use JsonCppConfig.cmake only if the file is present.
Merge pull request #10 from thewtex/Scale-sign

BUG: Scale cannot be negative in optimizer test.
BUG: Do not identify with parsed Json types during deserialization.

Use the known parameter types defined in the Serializer classes to determine
the type of entries during JSON deserialization.

JSON does not specify types, and recent JsonCpp does not necessarily add a
decimal to floats during write, so this solves the int/float ambiguity during
read.