Added a mailmap to clean up authors on rubygems

A mail map combines authors and changes their names so that it looks correct with `git shortlog`. I used it to clean up the ruby gems author list.
Is this correct? This would list you as "joshua-anderson", not "Joshua Anderson". Also, I think you can simplify the rules a bit:

```
<konstantin.mailinglists@googlemail.com> <konstantin.haase@gmail.com>
<me@henrikhodne.com> <henrik@hodne.io>
Joshua Anderson <j@zatigo.com>
Peter Souter <p.morsou@gmail.com>
```