If we want Tornado to be optional, as it can be, since we don't depend on the HTTP frontend for anything, then it shouldn't be included here.

If we want to make Tornado required just to get everyone to have a more complete Mopidy installation by default, then we should include it here, with a lower version, and remove it from the `extras_require` dict below. Then `'http': []` should be left there, so that `pip install Mopidy[http]` will still work.