Small doc/schema update
Documentation update
First attempt on http:// schema in images

You can request an image from an http server. The server downloads
the image and caches it in the configurable place $TMPDIR/sp/images.
On subsequent runs the image will be read from that place.

The caching works by taking the hostname and the requested part into
account
Bugfix: nested xpath functions

When running publisher.setup_page() during an active
xpath parsing session, everything gets dirty. The setup_page()
calls detect_pagetype, which runs the xpath parser and thus
messes up the state (m.Tok, m.Nextpos).

Now we save the state on a stack.

Next time we should not write global state dependent parser.
New xpath function sd:imageheight()

Returns the number of grid cells of the image height.
Schema fix: Element in ForAll