watching related units

[William Reade]

On Mon, 2012-06-11 at 09:49 +0200, William Reade wrote:
> However, I am now somewhat blocked on reviews. I have 2 pipelines open:
> 
> * version-watcher: https://codereview.appspot.com/6305070/
> * unit-relation-watcher: https://codereview.appspot.com/6300069/

This particular "pipeline" is now unit-relation-watcher alone, which no
longer uses VersionWatcher (which has been dropped entirely), and should
consequently be even more of a trivial pleasure to review ;).