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
I'm inclined to carry as little backward-compatibility baggage as possible into 1.0, so it'd be really nice to be free to reorganize built-in plugins at that point (for example, we can remove some source steps to plugins, break out some of the modules containing lots of steps into individual modules, and remove some of the compatibility code making, for example, `WithProperties` available at `buildbot.process.buildstep.WithProperties`.

Once we start maintaining full semantic-versioning style compatibility, we'll be adding a lot of *new* compatibility baggage, so removing as much *old* baggage as possible first is a good idea.

Anyway, we can say "will" and later decide not to!