Add mapping for myself

I got listed as `@MSch` for these commits: http://contributors.rubyonrails.org/contributors/msch/commits/in-time-window/today

And I'm "Martin Schürrer" here: http://contributors.rubyonrails.org/contributors/martin-schurrer/commits

I'd prefer to be "Martin Schuerrer" everywhere. This PR should do that I think.