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 talked about implementing these namespaces through dots in the entry_points names:
```
   'buildbot.util': [
     'repo.DownloadsFromProperties = buildbot.util.repo.RepoDownloadsFromProperties'
  ]
```
I think this makes it easier for the plugin author, and a bit clearer to read, too.