Refactor header and message constructors:

fix #581

* request and response headers now have convenience
  constructors so important fields like method, target,
  result, and version may be set upon construction.

Actions Required:

* Evaluate each message constructor call site and
  adjust the constructor argument list as needed.

example/server-framework/http_async_port.hpp

@@ -264,7 +264,7 @@ protected:
         // We construct the dynamic body with a 1MB limit
         // to prevent vulnerability to buffer attacks.
         //
-        parser_.emplace(1024 * 1024);
+        parser_.emplace(std::piecewise_construct, std::make_tuple(1024 * 1024));
 
         // Read just the header
         beast::http::async_read_header(