Use pkg_resource api to get config file

On OS X, installing from pip, the initial execution is broken when you run in any directory other than the one `__init__.py` happens to be in. One fix is to use `__file__` to find the config file; the setuptools people discourage this because you could be running from a zipfile or an egg or whatever. So `pkg_resources.resource_string` seems to be the way to go.