Add 512x512 version of icon to repository (Synergy Advance, b07475c)

Add the new, ultra-hi-res version of the icon to the repository in preparation for Leopard. The icon was kindly prepared by Adam "Elroy" Mazzitelli (<http://www.elroyonline.net/>).

Signed-off-by: Greg Hurrell <greg@hurrell.net>