Add support for pandoc-mode and remove for psgml/xml

Trying to simplify things for students:

- add support for pandoc mode and add to required packages
- remove psgml, xml modes
- remove support for tidy.el.
Add requirements section to README and clean links

- Add requirements (emacs 24)
- Correct links and change to reference-style links.
Remove clocking support from org-mode-setup

File not loaded by default anyway
Comment and wrapping edits only
Make mode line more user-friendly

- Add help-echo popups for items that did not yet have them
- Change "Mod" to "Unsaved" and don't bother showing for read only buffers
- Remove Emacs uptime from time popup
Bring in schwilk-theme edits from schwilk-emacs