MSVC CLI defined their own nullptr and provides for __nullptr for standard C++11.
https://msdn.microsoft.com/en-us/library/4ex65770.aspx
msgpack-c introduce MSGPACK_NULLPTR for internal use, it is defined as __nullptr only if compiled on C++ CLI otherwise defined as nullptr.
Added lacked include files.
Added a test.
Added MSVC build settings on appveyor.
Added old MSVC workaround.
Fixed a variable definition point to meet ANSI-C.