async cell execution

Hi,

It'd be nice to be able to execute async code in IPython cells - this can allow using IPython to develop e.g. asyncio code (cell = implicit asyncio coroutine) or Scrapy spiders.

I'm having this problem in Splash Notebook kernel (https://github.com/scrapinghub/splash/blob/master/splash/kernel/kernel.py). This kernel runs a QWebKit browser in the same event loop as IPython and allows to control it using Lua scripts. Lua commands use coroutines; it means in Python land they use callbacks or deferreds. For example, there is `splash:go(url)` command which loads an URL in a browser and waits until all resources are loaded; this is implemented by loading URL and setting a callback for 'loadFinished' event.

Currently this is implemented using a hack: `Kernel.do_execute` returns a Deferred which is fired when load is finished, and `Kernel.execute_request` adds a callback to this Deferred to send the result back, instead of sending the result immediately.

This mostly works, but there are hacks required to make `idle` / `busy` handled correctly because ipykernel thinks results are always returned immediately. Kernel.dispatch_shell is handled incorrectly by these hacks, so sometimes kernel shows 'busy' state while it should be in 'idle' state; this breaks autocompletion for some reason (is it disabled when kernel is 'busy'?).

Sorry for a brain dump :) I don't really understand what I'm doing: what is `dispatch_shell` method for, what most arguments mean (parent? stream? ident? etc.). I've sent https://github.com/ipython/ipython/pull/7713 in past to make IPython internals easier to work with for such async code, but never addressed the feedback. I think async support is most important for execute messages because they have real use cases; async completion and other messages are less useful, so it is fine to handle only execute messages.

I've seen https://github.com/ipython/ipykernel/issues/21 and https://github.com/takluyver/aiokernel, but didn't understood it at all; aiokernel look very different from what I came up with, so I have no idea what is it doing, or if it is solving the same issue or not.

Thoughts? Am I on a right track? Is this feature welcome? Is there a clean(er) way to implement it?