Added InfoExtractor type.
Added galleryList to Galleries.Retrieval.

galleryList downloads tree-like galleries which branch once at the top
level. That is, one function gets the targets to pursue (e.g. articles),
after which a list of functions is run on each target, with one node in
the list corresponding to one URL retrieval.

Additionally, galleryList extracts arbitrary local information (like the
title) from the root page.