Update supported Python versions

I propose:

- For 0.3.x, we update the docs to explicitly say we support Python 3.3 (we have in a practical sense for some time).

- For 0.4, we drop support for Python 3.1 (this recently became an issue in #2017, and we should try to limit the number of versions we support at all costs).

I don't (personally) think we should claim 3.4 support until the final release of 3.4 as things there are subject to change.

If we all agree, I'll create two separate pull requests for this.

EDIT: Updated reference to #2017.