Replaced the download_model method to the new one implemented in bob.extension

I've seen you are moving on with this package. That's cool. I will run it against IJB-C :-)

So, recently I implemented a feature in bob.extension that allows you to download and unzip files from a list of URLs in a particular directory.

I've patched that already in bob.ip.tensorflow_extractor

and in bob.ip.caffe_extractor

Would be nice to have it here.

What do you think?

Cheers