Print group callback

[aequitas]

This PR adds group callbacks to the group __str__ function.

>>> import dramatiq
...
... @dramatiq.actor
... def task():
...   pass
...
... group = dramatiq.group([task.message()])
... group.add_completion_callback(task.message())
... print(str(group))
...
group([task()]) -> (task())

[LincolnPuzey]

@aequitas Is there a particular need/reason for this change?

[aequitas]

I like to visualise generated tasks during debugging and this one was missing.