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

extras/beast/test/string_stream.hpp

@@ -8,6 +8,7 @@
 #ifndef BEAST_TEST_STRING_STREAM_HPP
 #define BEAST_TEST_STRING_STREAM_HPP
 
+#include <beast/core/async_completion.hpp>
 #include <beast/core/bind_handler.hpp>
 #include <beast/core/error.hpp>
 #include <boost/asio/buffer.hpp>