Tidy up core sources:

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

include/beast/core.hpp

@@ -0,0 +1,31 @@
+//
+// 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_CORE_HPP
+#define BEAST_CORE_HPP
+
+#include <beast/core/async_completion.hpp>
+#include <beast/core/basic_streambuf.hpp>
+#include <beast/core/bind_handler.hpp>
+#include <beast/core/buffer_cat.hpp>
+#include <beast/core/buffer_concepts.hpp>
+#include <beast/core/buffers_adapter.hpp>
+#include <beast/core/consuming_buffers.hpp>
+#include <beast/core/error.hpp>
+#include <beast/core/handler_alloc.hpp>
+#include <beast/core/handler_concepts.hpp>
+#include <beast/core/placeholders.hpp>
+#include <beast/core/prepare_buffers.hpp>
+#include <beast/core/static_streambuf.hpp>
+#include <beast/core/static_string.hpp>
+#include <beast/core/stream_concepts.hpp>
+#include <beast/core/streambuf.hpp>
+#include <beast/core/streambuf_readstream.hpp>
+#include <beast/core/to_string.hpp>
+#include <beast/core/write_streambuf.hpp>
+
+#endif