Add CONTRIBUTING.md
Merge branch 'develop'
Re-add `<em:type>` to install.rdf

It isn’t needed for extensions and themes, but AMO reviews strongly
recommend it anyway.
Add missing style rules
Improve coding style

- Almost all code is now wrapped at 80 chars, except a few special cases
  such as the `commands = [...]` block in commands.coffee. I didn’t
  bother to go through help.coffee and window-utils.coffee too much
  since they need rewrites anyway.
- Consistent use of parenthesis when calling functions.
- Consistent use of single quotes over double quotes.
- Consistent spacing.
- Removed unused or unnecessary code.
- Refactored unload.coffee into unloader.coffee, which should be easier
  to understand.
- Comments are now full sentences.
- This also fixes a few very minor bugs.

I’ve deliberately left out mode-hints/{hints,marker}.coffee to avoid
large merge conflicts with #330.