json-cpp Code

Fixed CMake / Unix build instructions.

- New CMake based build system. Based in part on contribution from
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).

- Patch #3393345: BOOST_FOREACH compatibility. Made Json::iterator more standard compliant, added missing iterator_category and value_type typedefs (contribued by Robert A. Iannucci).
- Patch #3474563: added missing JSON_API on some classes causing link issues when building as a dynamic library on Windows (contributed by Francis Bolduc).

Patch #3600941: Missing field copy in Json::Value::iterator causing infinite loop when using experimental internal map (#define JSON_VALUE_USE_INTERNAL_MAP) (contributed by Ming-Lin Kao).

Patch #3539678: Copy constructor does not initialize allocated_ for stringValue (contributed by rmongia).

Fix gcc -Wall warnings (patch from Matt McCormick)

Fixed warning(error?) on #if testing value of _MSC_VER without checking that it was defined.

Added missing "include/json/assertions.h" header in amalgamate.py.

Made it possible to drop null placeholders from array output.

This can be used when it's clear that the consumer is able to deal with
this, as web browsers are. Thanks to Yatin Chawathe for the patch.

Added an exit() to JSON_FAIL_MESSAGE to fix "no return" errors.