Bug 938222 - Show hash or other VCS identifier of the indexed source

See the discussion on bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=938222

## TODO

- [x] per-path hash and author lookup on Git
- [ ] per-path hash and author lookup on Hg
- [ ] per-path hash and author lookup on P4
- [x] VCS Hash on each compiled page (+ author)
- [x] VCS Hash on dir listings (+ author)


I'm still wrestling with which philosophy is best, but we at least should be consistent: metadata should either be generic metadata whose fields get no special mention in build.py, or we should go the whole way and make things like "author" and "revision" first-class entities with their own API hooks. Let me think about that.