Support Trollius

* Add GreenTrolliusTask, GreenTrolliusUnixSelectorLoop,
  GreenTrolliusEventLoopPolicy
* Use backported TestCase of Trollius on Python 2 to get assertRaises()
* Use exec() in test_tass.py because "yield from" raises a SyntaxError
  on Python 2
* New test_trollius.py testing the interoperability of Trollis and
  asyncio coroutines. greenio should support executing asyncio
  coroutines in a trollius event loop.