Automate gh-pages updates from master.

1) gh-pages content is now held in master doc/gh-pages/
2) scaled images for html docs are now version controlled rather than
scaled on-the-fly from the originals (these have to be stored on the
gh-pages branch and we want to avoid "new" binary files with each update).
3) new admin script to automatically update the gh-pages branch, and
optionally push to github, with said content and fresh html user guides.