Added a ValueType trait to TypeInfo.

This will allow for generic template functions that can deal with
complex types using thier underlying value types.  So rather than have:

template<T> T foo(const T&);
template<T> T foo(const std::complex<T>&);

to handle the case where an std::complex<float> should use a float
return value, you can now use a single signature:

template<T> TypeInfo<T>::ValueType foo(const T&);