Port greenio to Trollius

Hi,

I would like to use greenio in OpenStack to be able to use the asyncio API be execute tasks in greenlet. OpenStack projects are heavily based on eventlet and it's not possible to switch from eventlet to asyncio in one step, it should be done incrementally.

asyncio and Python 3 cannot be requirements on OpenStack because OpenStack is already deployed in production with Python 2, and projects don't support Python 3 yet (most clients are Python 3 compatibility, but not servers yet).

My plan is to use Trollius with greenio, replace incrementally eventlet with Trollius in OpenStack:
http://haypo-notes.readthedocs.org/openstack.html#roadmap

Victor
For the need: "yield from" syntax raises a SyntaxError on Python 2, you just cannot import the module.

It may be possible to use different files, but I prefer to keep all code in a single code base.