More subprocess leak fixes
Expose rejester.RejesterQueue() class
Create common py.test support module for rejester.

If you need a rejester "thing" (like a TaskMaster), declare
pytest_plugins='rejester.support.test', and you will have a task_master
fixture available to you that correctly deals with namespace management.

Speaking of namespaces, make sure every test has a unique namespace
(which solves some potential collision issues using xdist).