use urllib quote to encode username in culler FIX #573

[chicocvenancio]

Closes #573

[chicocvenancio]

Fwiw, I tested this and it kept working with usernames with ascii characters and started working with usernames with non-ascii characters chicocvenancio/paws@9a77d1d

[consideRatio]

I get this happy feeling about the code base when I think of how many is participating in testing it and contributing to it!

[minrk]

Thanks! This should be right. Since JupyterHub itself doesn't support Python 2 (3.5 for 0.9, 3.3 for 0.8), I don't think we need to support Python 2 in example scripts, but there's no harm in having an import check like this.