Send notifications for incoming messages.

Send notifications for incoming messages.

With this changeset EMchat will send notifications via libnotify's
`notify-send' for incoming messages.  It will only send the notification
if either the log buffer isn't the current buffer, or `emchat-frame' does
not have the window system focus.

	* emchat-log.el (emchat-notify-flag): New.  Send notifications
	when non-nil.

	* emchat-log.el (emchat-notify-command): Shell command used to
	send notifications.

	* emchat-log.el (emchat-notify): New.  Send notifications via
	notify-send from libnotify for incoming messages.

	* emchat-log.el (emchat-log): Send a notification if either the
	log buffer isn't the current buffer or the EMchat frame doesn't
	have focus.

Signed-off-by: Steve Youngs <steve@emchat.org>