Python 3.x support (using a single codebase approach)

Hi,

This pull request adds Python 3.x support to easy-thumbnails. All tests pass under python 2.6, 2.7, 3.2, 3.3 and pypy 2.0 beta1; python 2.5 support is dropped because exception handling syntax compatible with 2.x-3.x is not available under Python 2.5. Under Python 3.x a latest Pillow is required instead of PIL; I hope that it won't take a long time to get an official Python 3.x-compatible Pillow release.

Hello from PyCon Russia 2013 :)