Add subpackages to setup.py

When installing via pip, the sub-modules (distributions, ...) are not installed.
Some googling tells me that this might be because the sub-packages are not listed in the `setup.py`:

http://stackoverflow.com/questions/15368054/import-error-on-installed-package-using-setup-py
https://docs.python.org/2/distutils/examples.html#pure-python-distribution-by-package
