Prevent notifications from firing more than once per message

* riece-desktop-notify.el
(riece-desktop-notify-keyword-notify-function): Don't send
notification multiple times for a single message; don't wait until
the 'notify-send' program completes.
(riece-desktop-notify-last-message): New variable.
Suggested by Steve Youngs.
Include the message sender in desktop notification

* lisp/riece-desktop-notify.el
(riece-desktop-notify-title-function): Put the message sender's
nick in the title.
Add some eye-candy to desktop notifications

* lisp/COMPILE (riece-icons): Add riece-notify-icon.png.

* lisp/Makefile.am (EXTRA_DIST): Ditto.

* lisp/riece-desktop-notify.el (riece-desktop-notify-icon): New.
(riece-desktop-notify-gnu/linux-args): Add option to display icon.
(riece-desktop-notify-keyword-notify-function): Substitute the 'icon
symbol with riece-desktop-notify-icon.