separated session from session_impl and made a real pimpl-class of it. Introduced the possibility to destruct the sessions async.

docs/manual.rst

@@ -85,6 +85,8 @@ The ``session`` class has the following synopsis::
 			, bool compact_mode = true
 			, int block_size = 16 * 1024);
 
+		session_proxy abort();
+
 		void remove_torrent(torrent_handle const& h);
 
 		void disable_extensions();
@@ -156,6 +158,21 @@ returns. So, it's advised that any kind of interface (such as windows) are close
 destructing the session object. Because it can take a few second for it to finish. The
 timeout can be set with ``set_settings()``.
 
+abort()
+-------
+
+In case you want to destruct the session asynchrounously, you can request a session
+destruction proxy. If you don't do this, the destructor of the session object will
+block while the trackers are contacted. If you keep one ``session_proxy`` to the
+session when destructing it, the destructor will not block, but start to close down
+the session, the destructor of the proxy will then synchronize the threads. So, the
+destruction of the session is performed from the ``session`` destructor call until the
+``session_proxy`` destructor call. The ``session_proxy`` does not have any operations
+on it (since the session is being closed down, no operations are allowed on it). The
+only valid operation is calling the destructor::
+
+	class session_proxy { ~session_proxy() };
+
 
 add_torrent()
 -------------