Generate panorama figure using matplotlib

As discussed in https://github.com/scikit-image/scikit-image-paper/pull/58, it'd be nice to generate the panorama figure in matplotlib. This takes @jni's script from https://github.com/scikit-image/scikit-image-paper/pull/57 and modifies it generate a figure similar to the one composited by hand. I cherry-picked @jni's commit, which also modified a bit of pano.txt; let me know if I need to fix anything from that merge.