Add ability to register a user defined type conversion

Currently this adds a fair bit of overhead. It will need to be evaluated
further before it's merged.

include/chaiscript/dispatchkit/function_call_detail.hpp

@@ -17,7 +17,7 @@
 #include "boxed_cast.hpp"
 #include "boxed_number.hpp"
 #include "boxed_value.hpp"
-#include "dynamic_cast_conversion.hpp"
+#include "type_conversions.hpp"
 #include "proxy_functions.hpp"
 
 namespace chaiscript