Added API versioning.
Added a minimal forward declaration header file for pack/unpack overloading.
Separated adaptor functions' definitions and declarations.
Added packer_serializer class template declaration to object_fwd.hpp
Replaced tuple with std::tuple.
Added include file int_fwd.hpp for MSGPACK_ADD_ENUM.
Minimized header files dependency.
Added help comments for typical compile error.
Merge branch 'api_versioning' of https://github.com/redboltz/msgpack-c into redboltz-api_versioning

Conflicts:
	include/msgpack/unpack.hpp
Merge branch 'redboltz-api_versioning' into poc/0.6