fix imports for python 3

[djsutherland]

#54 apparently wasn't tested on python 3; it tries to do some relative import stuff, which doesn't work. This fixes it.

I also noticed that setup.py uses install_requires but doesn't actually use setuptools.setup, so that line only does anything if you're installing with pip and not if you're using setup.py directly. I changed it to use setuptools if available.

[martindurant]

Does this now need a from __future__ import for imports to work on py2?

[djsutherland]

Not in 2.7 (I tested). I think this syntax was introduced in 2.5, IIRC.

Edit: tested 2.6 and it works, too.

[martindurant]

All looks ok to me - anyone else with comments?