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Igor Okulist and Damien Buhl (Patch #14). Added support for running
tests and building with DLL on Windows.
- added missing JSON_API
- Visual Studio DLL: suppressed warning "C4251: <data member>: <type> 
needs to have dll-interface to be used by..." via pragma push/pop
in json-cpp headers.
- New header json/version.h now contains version number macros
(JSONCPP_VERSION_MAJOR, JSONCPP_VERSION_MINOR, JSONCPP_VERSION_PATCH
and JSONCPP_VERSION_HEXA). While this header is generated by CMake,
it is committed to ease build with alternate build system 
(CMake only update the file when it changes avoid issues with VCS).