Fixed issue #37.
It seems to be gcc's bug.
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58416
When the compiler is gcc, use memcpy, otherwise use an assignment operator.
The memcpy approach is a workaround for gcc's bug. The speed performance on gcc is the same between both approach. But on clang, the memcpy approach is 10% slower than an assignment approach. Hence I added the switching approach code using compiler checked macro.

Note: __GNUC__ is defined both gcc and clang. So I use __GNUC__ && !__clang__
Merge pull request #38 from redboltz/copy_msgpack_object_by_memcpy

Fixed issue #37.