This is a generic solution to the question of adding default reviewers.
It's generic enough to be useful both for installations like reviews.r-b.o
where there's a single group for all reviews, and the extraordinarily
complex case we've got at VMware, where different groups own different
parts of a single huge codebase.

There's a new model which basically has a regular expression and a list
of default people/groups.  When a new diff is created, it iterates through
the list of filenames in the diff and checks them against each of the
DefaultReviewer regexps.  If one matches, those defaults are added to the
draft's list of target reviewers.

Did a couple tests with different diffs to make sure it added the correct
reviewers.  This might blow up with diffs that contain added or removed
files, I'm not sure.  I'll do some more testing with that before I commit.