asyncio_slow: Add callback example from docs.
asyncio_slow: Add coroutine example from docs.
asyncio_slow: Add example for scheduling coro using Task.
asyncio_slow: Add example on chaining coros using "yield from" from docs.
asyncio_slow: run_until_complete() should not schedule anyhing.

Everything should be scheduled either already, or async() does this.
asyncio_slow: Implement loop.stop().
asyncio_slow: Add Future examples from docs.
asyncio_slow: Fix wait: again, should not schedule anything itself.
asyncio_slow: Add example of wait() from docs.