Don't use std::binary_function<>
Don't use std::binary_function<>

C++17 removes the previously deprecated std::binary_function, which was
already deprecated in C++11, so just don't use it at all, especially as
we don't really need it anyhow.

Closes #62.

vadz/xmlwrapp,vslavik/xmlwrapp,vslavik/xmlwrapp,vslavik/xmlwrapp,vadz/xmlwrapp,vadz/xmlwrapp