Added a celery worker that can be started as a thread

This worker can be used for testing purposes where using EagerResult does not suffice.

At the moment using this is VERY slow. The reason is that this implementation can not handle in-memory SQLite3 database. 

I have tried adding support for a shared memory sqlite3 database (LiveServerTestCase from Django does this), but I have not gotten it to work.

Hopefully someone with more skills than me can figure it out.