Apply corrections from the "include what you use" tool

Generally cleanups of the includes. Making sure each file
can properly stand on its own and forward declares when possible.

include/chaiscript/dispatchkit/exception_specification.hpp

@@ -7,8 +7,18 @@
 #ifndef CHAISCRIPT_EXCEPTION_SPECIFICATION_HPP_
 #define CHAISCRIPT_EXCEPTION_SPECIFICATION_HPP_
 
+#include <memory>
+
+#include "../chaiscript_defines.hpp"
 #include "boxed_cast.hpp"
 
+namespace chaiscript {
+class Boxed_Value;
+namespace exception {
+class bad_boxed_cast;
+}  // namespace exception
+}  // namespace chaiscript
+
 namespace chaiscript
 {
   namespace detail