Refactor the formula that calculates the distance between two points
Put crosshair and target circle on top of spirit level bubble

Remove from background image and put a new view on top
Set accelerometer background image to green if ball is inside small circle (otherwise red)
Fix bug where accelerometer updates stop when user goes to another view and comes back again

Start device motion updates in viewWillAppear instead of viewDidLoad
Fix bug where setting background image causes ball to jerk slightly

Removed background imageView and set view's backgroundColor property instead.