Tidy up core sources:

The core headers are moved to their own directory (but remain in
the same namespace).

include/beast/core/error.hpp

@@ -0,0 +1,24 @@
+//
+// Copyright (c) 2013-2016 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+
+#ifndef BEAST_ERROR_HPP
+#define BEAST_ERROR_HPP
+
+#include <boost/system/error_code.hpp>
+#include <boost/system/system_error.hpp>
+
+namespace beast {
+
+/// The type of error code used by the library
+using error_code = boost::system::error_code;
+
+/// The type of system error thrown by the library
+using system_error = boost::system::system_error;
+
+} // beast
+
+#endif