Add 'packages' entry to setup.py

[ggventurini]

Hi there,

I've been trying to try out pyautodiff with no luck, as

python setup.py install

does not copy the package files over to site_packages, resulting in errors such as:

    from autodiff import gradient
ImportError: No module named 'autodiff'

This pull request is to fix that, by adding a packages entry in setuptools' setup.

Cheers