green.yield_from() now accepts coroutine objects

To support asyncio and trollius in the same code base, asyncio.Task and greenio.GreenTask class should not be instanciated explicitly. To support Trollius, only Trollius tasks (trollius.Task or greenio.GreenTrolliusTask) should be used.

With the development versions of Trollius (which I plan to release under the version 1.0), there is now a loop.create_task() method which creates directly the best task object depending on the use case.

This patch changes greenio.yield_from() to support coroutine objects.

See also the pull request #5 which modifies greenio to support Trollius.