Add a .mailmap file

This file is used for example by command "git shortlog", to prevent duplicate
committers.

.mailmap

+# Map author and committer names and email addresses to canonical real names
+# and email addresses.
+#
+# For example with these commands:
+#   git shortlog -nse
+#   git shortlog -se | cut -f2 | sort
+
+Sébastien Helleu <flashcode@flashtux.org>