To support Trollius, the greenio project requires a "task factory" to create 
GreenTask instead of Task in Task._step(). In february, Luca Sbardella sent a 
similar request for its Pulsar project.

I propose to add get_task_factory() and set_task_factory() to event loop 
policy. The default task factory would be the Task class in asyncio, but the 
default may be different in a custom event loop policy.

Patch:
https://codereview.appspot.com/110820049/

See also the "Tulip, Trollius and greenio: add asyncio.tasks.task_factory" 
thread on the mailing list.