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/http/write.cpp

@@ -64,7 +64,7 @@ public:
         template<class ConstBufferSequence>
         std::size_t
         write_some(
-            ConstBufferSequence const& buffers, error_code& ec)
+            ConstBufferSequence const& buffers, error_code&)
         {
             auto const n = buffer_size(buffers);
             using boost::asio::buffer_size;
@@ -112,6 +112,7 @@ public:
             void
             init(error_code& ec) noexcept
             {
+                beast::detail::ignore_unused(ec);
             }
 
             template<class WriteFunction>