Adds zipcode.db to package_data in setup.py.

This allows for properly installing the package via a non-editable
git pip requirement. Previously, the db file was not kept with the
installed Python source when installed as a non-editable git pip requirement - e.g.:
git+git://github.com/joelcrocker/pyzipcode.git@09245d669a4bd3b9f2bf6c572a0ac93ca158a927#egg=pyzipcode

Not copying the zipcode sqlite database file along with the python sources into site-packages/pyzipcode meant that pyzipcode would be missing its database.