Add .mailmap file to get git-shortlog(1) right

The commit history contains several different email addresses for
Richard Hartmann, add .mailmap to map them all into one.

9d2da280-77a1-11e3-a114-001f3b596ec9
Merge remote-tracking branch 'sunny256/add-mailmap'

I don't see diverging histories locally, but that may be because of my
local setup.

No hurt in merging this, though.