A way to reference count (and thus delete) pieces.

Also some general housecleaning.
Refactoring aShelf to be its own module.
"Markup" post format

The idea is that the post links to attachments by content hash key
(i.e. root hash + depth) with metadata about the content as attributes
in the link tag. One link tag for all kinds of links, since getting "a
post plus attachments" is a common operation, so iterating through
"link" is better than through "video" and "audio" and "object" and
"embed" and etc.

serve.py doesn't work because http.server sucks and shouldn't be used.
No more examples again, also markup

I messed up the examples again because something fundamental needed
changing (I forget what). Also fiddling with markup, trying to figure
out how to get a HTML document tree that gives a general idea of how a
download is progressing.