Add 512x512 version of icon (Synergy, df70d19)

In preparation for Leopard, add a new, ultra-hi-resolution version of the application icon to the repository (not yet integrated into the build process). The update to the icon was kindly prepared by Adam "Elroy" Mazzitelli (<http://www.elroyonline.net/>).

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