Tidy up core sources:

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

test/core/to_string.cpp

@@ -0,0 +1,10 @@
+//
+// 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)
+//
+
+// Test that header file is self-contained.
+#include <beast/core/to_string.hpp>
+