Remove unused and obsolete classes and tidy up:

Many classes required to support type-erasure of handlers and boost::asio
types are now obsolete, so these classes and files are removed:
HTTPClientType, FixedInputBuffer, PeerRole, socket_wrapper,
client_session, basic_url, abstract_socket, buffer_sequence, memory_buffer,
enable_wait_for_async, shared_handler, wrap_handler, streambuf,
ContentBodyBuffer, SSLContext, completion-handler based handshake detectors.
These structural changes are made:
* Some missing includes added to headers
* asio module directory flattened

beast/module/asio/AsyncObject.h

@@ -0,0 +1,92 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of Beast: https://github.com/vinniefalco/Beast
+    Copyright 2013, Vinnie Falco <vinnie.falco@gmail.com>
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose  with  or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#ifndef BEAST_ASIO_ASYNCOBJECT_H_INCLUDED
+#define BEAST_ASIO_ASYNCOBJECT_H_INCLUDED
+
+#include <beast/Atomic.h>
+#include <cassert>
+
+namespace beast {
+namespace asio {
+
+/** Mix-in to track when all pending I/O is complete.
+    Derived classes must be callable with this signature:
+        void asyncHandlersComplete()
+*/
+template <class Derived>
+class AsyncObject
+{
+public:
+    ~AsyncObject ()
+    {
+        // Destroying the object with I/O pending? Not a clean exit!
+        assert (m_pending.get() == 0);
+    }
+
+    /** RAII container that maintains the count of pending I/O.
+        Bind this into the argument list of every handler passed
+        to an initiating function.
+    */
+    class CompletionCounter
+    {
+    public:
+        explicit CompletionCounter (Derived* owner)
+            : m_owner (owner)
+        {
+            ++m_owner->m_pending;
+        }
+
+        CompletionCounter (CompletionCounter const& other)
+            : m_owner (other.m_owner)
+        {
+            ++m_owner->m_pending;
+        }
+
+        ~CompletionCounter ()
+        {
+            if (--m_owner->m_pending == 0)
+                m_owner->asyncHandlersComplete ();
+        }
+
+    private:
+        CompletionCounter& operator= (CompletionCounter const&);
+        Derived* m_owner;
+    };
+
+    void addReference ()
+    {
+        ++m_pending;
+    }
+
+    void removeReference ()
+    {
+        if (--m_pending)
+            (static_cast <Derived *> (this))->asyncHandlersComplete ();
+    }
+
+private:
+    // The number of handlers pending.
+    beast::Atomic <int> m_pending;
+};
+
+}
+}
+
+#endif