add .mailmap

Clean up the various committer names and email addresses. For example, see
```
git log  --pretty="format:%an <%aE>" | git check-mailmap --stdin | sort -u
```
to get a clean list of all contributors to Prosa. 

.mailmap

+Björn Brandenburg <bbb@mpi-sws.org> Björn Brandenburg <bbb@mpi-sws.org>
+
+Sergey Bozhko <sbozhko@mpi-sws.org> Sergei Bozhko <sbozhko@mpi-sws.org>
+Sergey Bozhko <sbozhko@mpi-sws.org> Sergey Bozhko <gkerfimf@gmail.com>
+Sergey Bozhko <sbozhko@mpi-sws.org> Sergey Bozhko <sbozhko@sws-mpi.org>
+
+Kimaya Bedarkar <kbedarka@mpi-sws.org> Kimaya <f20171026@goa.bits-pilani.ac.in>
+Kimaya Bedarkar <kbedarka@mpi-sws.org> Kimaya Bedarkar <f20171026@goa.bits-pilani.ac.in>
+Kimaya Bedarkar <kbedarka@mpi-sws.org> kbedarka <kbedarka@mpi-sws.org>
+Kimaya Bedarkar <kbedarka@mpi-sws.org> kimaya <f20171026@goa.bits-pilani.ac.in>
+Kimaya Bedarkar <kbedarka@mpi-sws.org> Kimaya <kbedarka@mpi-sws.org>
+
+Marco Maida <mmaida@mpi-sws.org> Marco Maida <>
+Marco Maida <mmaida@mpi-sws.org> mmaida <mmaida@mpi-sws.org>
+
+Mariam Vardishvili <mariamvardishvili@gmail.com> mariamvardishvili <mariamvardishvili@gmail.com>
+Mariam Vardishvili <mariamvardishvili@gmail.com> mariamvardishvili <mvardishvili@mpi-sws.org>
+Mariam Vardishvili <mariamvardishvili@gmail.com> Mariam Vardishvili <mvardishvili@mpi-sws.org>
+
+Maxime Lesourd <maxime.lesourd@inria.fr> Maxime Lesourd <maxime.lesourd@gmail.com>
+Martin Bodin <martin.bodin@inria.fr> Martin Constantino–Bodin <martin.bodin@ens-lyon.org>
+Martin Bodin <martin.bodin@inria.fr> Martin Constantino–Bodin <martin.bodin@inria.fr>
+Pierre Roux <pierre.roux@onera.fr> Pierre Roux <pierre@roux01.fr>
+Sophie Quinton <sophie.quinton@inria.fr> sophie quinton <squinton@inrialpes.fr>
+Sophie Quinton <sophie.quinton@inria.fr> sophie quinton <sophie.quinton@inria.fr>
+Xiaojie GUO <xiaojie.guo@inria.fr> Xiaojie GUO <gxj0303@loic.inrialpes.fr>
\ No newline at end of file