PyClaw gallery working again

This PR updates the gallery functionality to be compatible with changes over the last year.  The gallery can now be built by:

    cd pyclaw/doc/gallery
    python make_plots.py
    python gallery.py

Also, if you are deploying the gallery to the web, you should copy your apps/ directory to doc/_built/html first.  Then

    cd pyclaw/doc
    make html
    make deploy

It would be nice to update the gallery to include some new examples, but I have not done that yet.  The current gallery can be seen at http://numerics.kaust.edu.sa/pyclaw/gallery/