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.

dkimpy_milter/__main__.py

@@ -0,0 +1,6 @@
+#!/usr/bin/python2
+
+from dkimpy_milter import main
+
+if __name__ == "__main__":
+    main()