Added two new datasets online and updated fetcher.py. 

The first is a 3-shell HARDI dataset and the second is a 2-shell phantom dataset.

This PR is in support of @smerlet and @ecaruyer work on continuous modelling to be used with their tutorials.
 
@stefanv at some point suggested to add md5 checksums in fetcher.py in order to check the validity of the downloads. I think it is a good idea. Any volunteers? @stefanv can you show us how to do that?