Apply results of running 'python-modernize'.

As well as additional fixups on the way toward "stradding" Python2 / Py3k.
Make dependency on 'six' explicit.
Merge pull request #13 from tseaver/apply-modernize-prep-py3k

Apply results of running 'python-modernize'.
Make one tweak from python-modernize.
Simplify import generation for generated clients.

There's a complex web of trickery in place for how I write import lines in
generated clients, mostly to make things work both internally and externally.
At this point, it's just confusing -- simplifying for now and I'll deal with
merging it internally later.
Add defaults for client ID/client secret.