json-cpp Code

Added clang compiler for continuous integration.

Added basic Travis CI integration contributed by Igor Okulist.

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.