Initial support for Python 3

Hey Chris, 

Here's Python 3 support for Kiwi, tested on 2.7, 3.3.2 and 3.4a4 on OS X Mavericks and a gentoo build.

I realise the project is mostly WIP right now, but it'd be good to add py3 support :) 
i've left the base kiwi solver code alone and just changed the python wrapper to do the bytes/str conversions. this seemed more sane than attempting to change the kiwi solver to use unicode! 

also, have added a *very* basic unit test to create a solver and add constraints - this proved really useful when getting the build working for 2.x and 3.x, however i stopped short of putting in an entire test suite, as i assumed you're still playing with the solver code :)

am more than willing to put in numerical tests when you're happy with the solver.

cheers,
dave