create_task requires an assignee

[dmoskov]

This is the culprit:
payload = {'assignee': assignee or 'me', 'name': name,

That makes it impossible to create a task w/o an assignee, which is a common case

[GRArmstrong]

I agree, especially since if you create a task with the create_task() method, the default action is to not assign anybody, I don't feel it makes sense for the two different methods to act in different ways.

It'd be nice to add some of the functionality of create_task() into create_subtask()