adding a .mailmap file

This give improved legibility when using git shortlog by combining multiple
usernames/emails into a single line:

```shell
$ git checkout master
Switched to branch 'master'
$ git shortlog -s -e -n
    31  Francesc Alted <faltet@pytables.org>
    20  Valentin Haenel <valentin.haenel@gmx.de>
    14  Francesc Alted <francesc@continuum.io>
     7  FrancescAlted <francesc@continuum.io>
     2  FrancescAlted <faltet@pytables.org>
     1  cgohlke <cgohlke@uci.edu>
     1  gcmalloc <gcmalloc@gmail.com>
$ git co mailmap
Switched to branch 'mailmap'
$ git shortlog -s -e -n
    54  Francesc Alted <francesc@continuum.io>
    21  Valentin Haenel <valentin.haenel@gmx.de>
     1  cgohlke <cgohlke@uci.edu>
     1  gcmalloc <gcmalloc@gmail.com>
```

The discrepancy in my number of commits is caused by the commit that introduces
the mailmap.
Merge pull request #21 from esc/mailmap

Adding a simple mailmap file