meta.h: add IsBaseOf meta function
meta.h: add IsBaseOf meta function

In order to match GenericValue and its derived classes for the
SFINAE-implementation of some of the operators/functions, this
meta-function matches all types equal to or derived from a given
class.  See std::is_base_of<B,D> available in C++11.

Define RAPIDJSON_HAS_CXX_TYPETRAITS to use the C++11 implementation.

zhengxle/rapidjson,zhengxle/rapidjson,zhengxle/rapidjson