Introduce global timed handlers

There are situations when applications need more predictable timed
handlers that don't depend on connection status. Other usecase of
the global handlers is to manage offline connections, for example,
reconnect after a disconnection.

ChangeLog

@@ -3,6 +3,9 @@
 	- SCRAM-SHA-256 and SCRAM-SHA-512 support
 	- c-ares support
 	- LibreSSL support
+	- Introduced global timed handlers that fire periodically regardless of
+	  connections status, such a handler can be used to implement deferred
+	  re-connection
 	- examples/register implements XEP-0077
 	- Fixed issue with IPv6 on Windows (#153)
 	- Improved portability across systems such as Haiku, Windows
@@ -13,6 +16,8 @@
 		- xmpp_conn_is_disconnected()
 		- xmpp_stanza_add_child_ex()
 		- xmpp_stanza_get_context()
+		- xmpp_global_timed_handler_add()
+		- xmpp_global_timed_handler_delete()
 
 0.9.3
 	- PLAIN mechanism is used only when no other mechanisms are supported