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/language/chaiscript_algebraic.hpp

@@ -7,6 +7,8 @@
 #ifndef CHAISCRIPT_ALGEBRAIC_HPP_
 #define	CHAISCRIPT_ALGEBRAIC_HPP_
 
+#include <string>
+
 #include "../dispatchkit/dispatchkit.hpp"
 
 namespace chaiscript