anchor: Joystick:isDown
  Checks if a button on the Joystick is pressed.
  
  LÖVE 0.9.0 had a bug which required the button indices passed to Joystick:isDown to be 0-based instead of 1-based, for example button 1 would be 0 for this function. It was fixed in 0.9.1.
  anyDown = Joystick:isDown( buttonN )
  buttonN number The index of a button to check.
  anyDown boolean True if any supplied button is down, false if not.