Added .mailmap.

This file is used by e.g. git-shortlog to map author E-mail addresses to real
author names. The file currently includes mappings for the old SVN logins as
well as a few cases where people where using different E-mail addresses.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>