adapt code, test and doc to latest dstore

Please note that this requires dstore master, not "just" dstore 1.1 (in particular because Observable has been renamed to Trackable).

This supersedes #198 and also solves #187.

cc: @sbrunot 
I'm not sure. I was basically waiting for your feedback on that one. I guess my aim was more to make it protected than public so that you can on your side call `fetchRange` instead of `fetch` in it, but maybe this is not fulfilling your needs?