Removed C++11 features

C++11 features are used on the following commit:
https://github.com/msgpack/msgpack-c/commit/97a7b7545a529bc29e1cb444b446ccac9883b316
http://www.cplusplus.com/reference/type_traits/is_signed/?kw=is_signed

I got a compile error when I did 'make check' when I configured with CXXFLAGS="-std=c++03".

#include <limits> is not mandatory because other header files include that. But it helps readability of the code.
http://www.cplusplus.com/reference/limits/numeric_limits/?kw=numeric_limits