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
- [x] update documentation
  - [x] user documentation
  - [x] packager documentation
(A random user/interested party appears) FWIW, do note that docstrings allow your editor/IDE to display helpful tips about the (public) function you're about to use. I can hardly imagine Python code as useful without docstrings, to be honest. In my opinion, it's much easier to keep docstring-style code up-to-date, than some external documents.