Reduce cost of including chaiscript.hpp

- ChaiScript no longer includes or automatically instantiates std lib
 - ChaiScript constructor now requires an std lib instance in the form
   of a ModulePtr object
 - This new layout facilitates better usage of compilation firewalls and
   factories for reducing the overall impact of ChaiScript on a project

include/chaiscript/language/chaiscript_common.hpp

@@ -7,6 +7,7 @@
 #ifndef CHAISCRIPT_COMMON_HPP_
 #define	CHAISCRIPT_COMMON_HPP_
 
+#include <sstream>
 #include "../dispatchkit/dispatchkit.hpp"
 
 namespace chaiscript