NF: Downloader for the Localizer dataset.

Downloader for the Localizer dataset from http://brainomics.cea.fr

This is WIP because the following points still have to be addressed:
- provide an example (`plot_localizer_auditory(?)_mass_univariate.py`, ongoing)
- add tests for the downloader

What can be done by reviewers in order to help:
- check that the changes introduced in downloading tools are ok and not too specific.
- help finding good names to wrap Brainomics contrast names.
- help choosing the simpler caller functions that can be found at the end of `datasets.py`

For all these points I put more detailed comments below, directly in Github's code viewer.