Made XKCD script more robust.
Fixes #63.
Fixed Fetch.Forest

The handling of leaves and inner nodes was quite bungled.

1) Inner nodes received UUIDs, not the leaves (fixed now).
2) Inner nodes without successors were not handled in 224 (leaves').
They were a perfectly possible case if the fetch process ran into a dead
end; they are now correctly recognised and thrown away, since they are
not needed.