Added default implementation to string2type::convert function.

examples/demo.cpp

@@ -2,17 +2,6 @@
 
 #include <iostream>
 
-#include <boost/lexical_cast.hpp>
-
-namespace string2type {
-
-template<>
-int convert<int>(const std::string &value) {
-    return boost::lexical_cast<int>(value);
-}
-
-}
-
 struct foo : attributes {
     foo() {
         REGISTER_ATTRIBUTE(int, x);