Tidy up core sources:

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

include/beast/core/handler_concepts.hpp

@@ -0,0 +1,27 @@
+//
+// 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_HANDLER_CONCEPTS_HPP
+#define BEAST_HANDLER_CONCEPTS_HPP
+
+#include <beast/core/detail/is_call_possible.hpp>
+#include <type_traits>
+
+namespace beast {
+
+/// Determine if `T` meets the requirements of @b `CompletionHandler`.
+template<class T, class Signature>
+#if GENERATING_DOCS
+using is_CompletionHandler = std::integral_constant<bool, ...>;
+#else
+using is_CompletionHandler = std::integral_constant<bool,
+    std::is_copy_constructible<typename std::decay<T>::type>::value &&
+        detail::is_call_possible<T, Signature>::value>;
+#endif
+} // beast
+
+#endif