Add Message tray button on the right of the bottom panel (zpydr/gnome-shell-extension-taskbar pull #14)

Added Message tray button on the right of the bottom panel (see https://github.com/zpydr/gnome-shell-extension-taskbar/pull/14 )

In function bottomPanel I've added empty_space because I don't know how to put MessageButton on the right - this is the first time when I'm coding gnome-shell extension :)
TODO: check this code on Gnome Shell version 3.8 and 3.10 - it works fine on ver. 3.4, but I didn't tested on newer releases.

MessageButton function copied from Bottom_Panel extension (licence GPLv2+), see
http://intgat.tigress.co.uk/rmy/extensions/index.html