Adds the python package `face_recognition` and its dependency
`face_recognition_models`.

This package is a wrapper for `dlib` which is able to detect faces in a
given image. I checked the package with the following expression:

```
with import ./. { };

stdenv.mkDerivation {
  name = "facetest";
  src = null;
  buildInputs = with pythonPackages; [ face_recognition ];
}
```

The package works perfectly fine in a `nix-shell`:

```
$ nix-shell
[nix-shell:~]$ python
Python 2.7.14 (default, Sep 16 2017, 17:49:51)
[GCC 7.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import face_recognition
>>> img = face_recognition.load_image_file("/home/ma27/me.jpg")
>>> print(face_recognition.face_locations(img))
```

pythonPackages.face_recognition: init at 1.2.1