use py.test as testrunner

changes the test-runner to py.test (see #8 and #1).  (no import errors any more)

open for now: 
- docs for this? 
- test design itself (where to put the test-app and the models?)


I removed the test-settings from the old test-runner, I would use a _real_ settings-file and set it in `[pytest]` in the `tox.ini`