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/core/impl/basic_streambuf.ipp

@@ -8,6 +8,7 @@
 #ifndef BEAST_IMPL_BASIC_STREAMBUF_IPP
 #define BEAST_IMPL_BASIC_STREAMBUF_IPP
 
+#include <beast/core/detail/type_traits.hpp>
 #include <beast/core/detail/write_dynabuf.hpp>
 #include <boost/assert.hpp>
 #include <algorithm>
@@ -786,6 +787,7 @@ void
 basic_streambuf<Allocator>::
 copy_assign(basic_streambuf const& other, std::false_type)
 {
+    beast::detail::ignore_unused(other);
 }
 
 template<class Allocator>