Mercurial Bookmarks and Bug Fixes

Fixed a few bugs in the mercurial extractor, as well as adding support for basic handling of bookmarks.

Most of it is pretty tame, but you'll want to take a look at the way I sanitized the author names. Reposurgeon doesn't work well when there's no author email, but mercurial repos can absolutely have all sorts of crazy stuff in the author field. Mercurial already has routines to try to sort it semantically, so I farmed the job out to them. I had to update the unit tests on that one, though, since names will no longer be printed with quotes.

It also includes some changes for handling subrepositories in mercurial. Those are gnarly and there's no really decent way to handle them. The best I've been able to come up with is ignore them in a moderately resilient way. There are performance costs if your repo is full of busted subreo links, but should be no cost if your repo isn't.