fix tornado log propagation

in the notebook
bump minimum tornado version to 3.1.0

The no-hidden behavior of the files handler
already requires this, but instead of raising, it just doesn't hide hidden files.

Bumping the minimum version also allows much cleaner static file handler subclasses.
Merge pull request #4428 from minrk/tornado-3.1

bump minimum tornado version to 3.1.0