Move celery serializer to json, change classes into class names

NOTE: Do not merge!  query (http response not serializable) does not work yet

@martijnbastiaan I think it is generally desirable to set celery to use json for all (de)serialization right? That makes it safer, more transparent what gets passed around, and in Task we assume that the called_with is json serializable anyway. 

Problems: 
- codingjob export needs debugging
- queries return http response, should be fixed