watching related units

[William Reade]

On Mon, 2012-06-11 at 14:22 +0200, William Reade wrote:

OK, branch state is now as follows:

Small and self-contained:

* unit-relation-watcher https://codereview.appspot.com/6305082
* tweak-children-watcher https://codereview.appspot.com/6301066

Pipeline:

* convenient-relation-interface: https://codereview.appspot.com/6303060/
* add-unit-relation: https://codereview.appspot.com/6304057/
* related-units-watcher: WIP; depends also on the two small branches
above.

...so: I would most appreciate reviews of the two small branches, so I
can merge them through the pipeline and reconcile my parentage; we can
then work though the pipeline at leisure. Sorry for all the noise.

Cheers
William