Fixes #88: Linking error on MacOS Mavericks

The c++ library implementation on MacOS is broken, it does not allow you to
correctly use points to members of std::string. We work around this by not
directly using member pointers and instead wrapping the method calls
with our own functions.

include/chaiscript/dispatchkit/boxed_number.hpp

@@ -10,6 +10,7 @@
 #include "boxed_value.hpp"
 #include "../language/chaiscript_algebraic.hpp"
 #include <boost/cstdint.hpp>
+#include <sstream>
 
 namespace chaiscript 
 {