Add compat for Python 2.7/3.2 + Django 1.4/1.5, passes tests.

Dear people,

This pull requests provides sorl-thumbnail with compatibility for Python 2.7/3.2 and Django 1.4/1.5.

Passes tests under:
* Python 3.2 + Django 1.5.1
* Python 2.7 + Django 1.5.1
* Python 2.7 + Django 1.4.5

It include the minimum necessary changes to provide the compatibility, no extra condiment. Almost certainly won't work under Django 1.3 so a version bump would probably be a good idea.

[Pillow](https://pypi.python.org/pypi/Pillow/2.0.0) replaces PIL under Python 3 in order to have the same functionality as the latter under Python 2.

Comments and feedback are welcome.