change to jquery object naming convention

prefix jquery object variables with $
change arrow color to silver when it is clicked

-change arrows from <img> to <canvas> so that pixel data can be
manipulated
polish arrow color changing behavior

- detect whether mouse is hovering over the arrow image itself, and not
any other part of the canvas.
- turn arrow gold when hovered over.
- register a mouse click only if the mousedown and mouseup both occurred
while hovering over the arrow