Add support for Python 3.3.
Fixes documentation, setup.py to reflect Python 3 changes.
Updates development version to 0.13.0-dev.

Increasing the minor revision number because this version will be backwards
incompatible: in order to add Python 3 support, we require Flask 0.10 or
greater. With Flask 0.10, support for Python 2.5 is no longer required for
extensions (like Flask-Restless).