Add Message tray button on the right of the bottom panel (see Issue #11)

Message button feature added by Jurgis Pralgauskis <jurgis.pralgauskis@gmail.com>
MessageButton function copied from Bottom_Panel extension: 
http://intgat.tigress.co.uk/rmy/extensions/index.html 

In function bottomPanel added empty_space because I don't know how to put MessageButton on the right - this is my first code for gnome-shell extension :)