Basic fixes with 2to3 -wn; added __repr__ to models; read bytes instead ...

Added __repr__ to models; read bytes instead of strs except when opening zip files.
Basic fixes with 2to3 -wn

The bytes fix in cities/management/command/cities.py isn't so dry, but I couldn't figure out how to test the type of stream inside the generator. Maybe you know a way?
What's the purpose of the file.mode check? Both branches look identical.