Enable "custom" tostring operators

[rpavlik]

In cases where an existing operator<< doesn't exist or is unsuitable for use in this way, it would be good to be able to optionally pass a function, taking a (reference/const reference/pointer/pointer to const) and returning a string.

Right now this can kind of be "hacked" into place with something like this above the binding:

namespace luabind {
    static inline std::string tostring_operator(CLASSNAME const& val) {
         /// your code here
         return s;
    }
}

which will be preferentially selected over the function template in operators.hpp, but it's not pretty or particularly expressive.