Pretty up the last plot in the example notebook

Mainly, I wanted to make the last plot in the notebook prettier. I also made the pylab import explicit, and set the origin of image plots to be lower so the notebook will look the more or less the same if/when users run it on their own machines.

(It appears that axis tick labels don't change with the `image.origin` setting, leading to ambiguous plots. We should perhaps change that in poppy.)