Cleaned up pointer and reference versions of unpack().
Removed obsolete unpack interface from test codes.
Replaced size_t with std::size_t.
Replaced C-Style cast with static_cast.
Use const char* version of msgpack::unpack_error to avoid std::string memory allocation when usr C++11 compiler.
See http://www.cplusplus.com/reference/stdexcept/runtime_error/?kw=runtime_error
Added reference version of unpacker::next().
Updated test codes using recommended interface.
They used to use obsolete interfaces.