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

test/websocket/websocket_sync_echo_server.hpp

@@ -8,6 +8,7 @@
 #ifndef BEAST_WEBSOCKET_SYNC_ECHO_PEER_H_INCLUDED
 #define BEAST_WEBSOCKET_SYNC_ECHO_PEER_H_INCLUDED
 
+#include <beast/core/placeholders.hpp>
 #include <beast/core/streambuf.hpp>
 #include <beast/websocket.hpp>
 #include <boost/optional.hpp>
@@ -36,7 +37,7 @@ private:
     std::thread thread_;
 
 public:
-    sync_echo_server(bool server, endpoint_type ep)
+    sync_echo_server(bool /*server*/, endpoint_type ep)
         : sock_(ios_)
         , acceptor_(ios_)
     {