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 :)
Added message-button style for MessageButton function (pull #14)

Added message-button style for MessageButton function (see https://github.com/zpydr/gnome-shell-extension-taskbar/pull/14 ), copied from Bottom_Panel extension (licence GPLv2+): 
http://intgat.tigress.co.uk/rmy/extensions/index.html
Merge pull request #14 from Baltix/patch-1

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