Code review comment for lp:~allenap/launchpad/subscribe-to-tag-bug-151129-5

Thanks for the review!

In summary I've:

- used PersonSet()._getPrecachedPersons() -- it's awesome :)

- added docstrings to the *Set classes.

- added a note about the XXX to BRANCH.TODO. Still not sure what I'm
  going to do about it.

- removed the XXX about the bug supervisor. The bug supervisor gets
  enough mail as it is.

- given test_person_sort_key a big overhaul, and your layer suggestion
  makes it run much faster, thanks.

If you're interested, there's a diff of the changes at:

  http://paste.ubuntu.com/546982/

Nothing controversial though.