Fix unused parameter warnings and missing includes:

fix #127

* Added beast::detail::ignore_unused based on boost::ignore_unused
* Added -Wextra compilation flag when building with gcc
* Fixed all unused parameter warnings with ignore_unused
* Fixed all missing includes when building each .hpp separately

include/beast/http/string_body.hpp

@@ -9,6 +9,7 @@
 #define BEAST_HTTP_STRING_BODY_HPP
 
 #include <beast/http/body_type.hpp>
+#include <beast/core/detail/type_traits.hpp>
 #include <boost/asio/buffer.hpp>
 #include <memory>
 #include <string>
@@ -73,6 +74,7 @@ private:
         void
         init(error_code& ec) noexcept
         {
+            beast::detail::ignore_unused(ec);
         }
 
         std::uint64_t