.mailmap: Add mappings to consolidate author names and emails

See "MAPPING AUTHORS" in git-shortlog(1) for syntax details.

I don't think I've accidentally merged two distinct people together, but I
added

  Guillaume B <guitreize@gmail.com>

to avoid someone accidentally merging him into

  Guillaume <guillaume@lame.homelinux.com>

I'm pretty sure these are two different Guillaumes.