Set up __main__.py, use it in tests

This allows us to invoke dkimpy-milter as "python -m dkimpy_milter
dkimpy-milter.conf", which makes running the test suite easier.

tests/dkimpy-milter

@@ -0,0 +1,2 @@
+#!/bin/sh
+python2 -m dkimpy_milter "$@"