Always include cwd in import paths

[asottile]

If #208 doesn't get changed upstream, we can solve it here.

I removed the documentation about passing in str for include_paths, it never worked due to the following:

>>> isinstance('hi', collections.Sequence)
True
>>> os.pathsep.join('hi')
'h:i'

I also removed the custom TypeError (just allowing tuple(...) to raise)

[asottile]

I'm going to merge this for now, if upstream adjusts we can too -- want to fix our "latest" release though :)