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
`plugins` should be here, for sure.  And the `customization` page should talk about writing plugins.  But I think the actual `plugins-dev` file belongs in `master/docs/developer`.  The `customization` page already links there, which I think is sufficient connection.  I definitely don't think plugin development deserves its own top-level chapter in the manual.