Rename to http::dynamic_body, consolidate header:

fix #284

* streambuf_body is renamed to dynamic_body
* basic_dynabuf_body is renamed to basic_dynamic_body

examples/http_example.cpp

@@ -34,7 +34,7 @@ int main()
 
     // Receive and print HTTP response using beast
     beast::streambuf sb;
-    beast::http::response<beast::http::streambuf_body> resp;
+    beast::http::response<beast::http::dynamic_body> resp;
     beast::http::read(sock, sb, resp);
     std::cout << resp;
 }