Use of setuptools instead of distutils.

Hi,

Thank you for the amazing work.

Here is a very dumb and simple patch to use setuptools instead of distutils.

This is especially useful when generating a Debian package (which I actually done [here](https://github.com/freelan-developers/flask-gravatar.debian) for Flask-Gravatar) because distutils doesn't deal nicely with the fact that the `__init__.py` file in `flaskext` is provided by `Flask` itself.