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.
Fix test_task_yield_from_invalid()

The exact error message depends on the version of Trollius and Tulip, if
they have the new create_task() function.

The check may be removed later when Trollius and Tulip with
create_task() will be widely deployed.
Split test_tasks.py in two parts

* Add a new test_asyncio_trollius.py file to test asyncio and trollius
  interoperability
* Move task tests for trollius into test_tasks_trollius.py
* Remove unused "import socket" in test_socket.py
Cleanup tests
Run test_socket on asyncio and trollius
Also use loop.create_task() with asyncio
Merge pull request #9 from haypo/trollius2

Support Trollius