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
Editor help strings is a good argument, but I do have to disagree with the ease of updating -- Buildbot was an awful mess when we were trying to do both!

When we get to Buildbot-1.0.0, and there are some nice, stable public APIs, then we can go back and re-add docstrings for those APIs (probably in some "safe" way such as only in the interfaces).  We can also auto-generate the narrative documentation *from* those docstrings, reducing the WETness :)