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
We've had issues with users finding apidocs or, even worse, docstrings in the code and treating them as documentation, yet they are often incorrect or documenting internal APIs.

This is much less of a problem now that we're not automatically generating apidocs, so it's probably OK to use them here -- but we've been avoiding them in new code for some time now.