Added unpublishing test and moved singal processor

[jakob-o]

Hey,

I added a test for the unpublishing. I was able to run it successfully with elastic search as a backend (solr needed configuration, simple engine does neither supports removal nor models()). Also had to move the signal processor to a separate module, otherwise the tests would fail due to circular imports. Hope this makes sense

Regards

Jakob

[czpython]

Awesome! I'll check it out in a bit :)