Add future/greenlet adaptors

[fantix]

Within asyncio code, we should be able to wait for greenlets like this:

@asyncio.coroutine
def some_routine():
    yield from as_future(gevent.spawn(....))

Likewise, we should also be able to wait for futures in gevent code:

def gevent_handle():
    wait_future(asyncio.sleep(1))

At last, we want to mix explicit asyncio yields and implicit gevent switches in one method:

@tulipcore.coroutine
def some_routine():
    yield from asyncio.sleep(1)
    gevent.sleep(1)

[graingert]

This would be extremely handy. I'd like to see some gunicorn demos soon!

[fantix]

https://github.com/decentfox/tulipcore/tree/coroutine