50 shades of face

In this MR I created a directory containing a repository of notebooks (so far only one) with some nice features that doesn't fit in the API.

For instance, I added a notebook called 50-shades-of-face to evaluate the best way to crop a face given a dataset and a feature extractor. Some people already questioned how we pick those values, so I pasted an example with Mobio.

People can take this notebook and customize as they like.