Fix error with package setup

When `setup.py install` is called, subpackages are not installed and `pint`
fails to import in Python. Using the `find_packages()` method solves that
by looking for all packages and subpackages and adding them to the install procedure. I have tested it and it seems to work. Now I can use `pint` on my machine (Windows 7).

This should also solve the error I got when trying to install the package using `pip install pint`: the subpackages were missing.