#5 optional axes

[Toby Donaldson]

Add a feature that allows the user of TheMatrix to
display x an y axes on the image. Arrows at the end of
the axes should show in what direction the coordinate
values increase. Tick-marks along the axis can also be
added every 10 pixels, say.

Note that this feature is a purely visual one, and it
is meant only to display the underlying coordinate
system, not change it.