* Generate medium sized pages correctly when not generating full size pages.
* Allow a fixed height to be specified instead or as well as a fixed width, in
  the latter case, resize to which ever fits the bounding box, but keep the
  aspect ratio.
* Allow ordering of images rather than the current "sort" based algorithm on
  filename.
* Make sure that the tools we require are available (i.e. check that we have
  xargs, convert, identify etc)