There's already a Shake Shack iPhone app that lets you look at the burger joint's line via live camera, so you know before you go over there just how much bitching on Twitter you will do while waiting for a sorta decent burger in Madison Square Park. Now there is an app, we have been notified, called Shakedown (that is an iTunes store link! This is their website) that will sync with FourSquare (the fun/ridiculous/scary app that tells your "friends" where "you are") so as to ID your friends already in line so that you can therefore jump in and butt ahead of plebes who are not [...]