Plugins

Docs is still missing.  Basic infrastructure is in place.

All kind of comments are more than welcome :)

- [x] basic infrastructure
- [x] adopt it for built in stuff
  - [x] www
  - [x] slaves
  - [x] change sources
  - [x] schedulers
  - [x] steps (seems the most "interesting")
  - [x] status receivers
  - [x] utilties
- [x] provide unit tests
- [x] update release notes
- [ ] update documentation
  - [x] user documentation
  - [ ] packager documentation
It's a minor point, but in new documentation we're writing one sentence per line.
That makes diffs easier to read, since paragraphs don't get re-wrapped when a single word is added or removed.