Add a simple example to show how to create a client that gathers multiple web resources _concurrently_ .

I am not sure if this example is serial or concurrent.  :-(  If it is serial then it would be quite helpful for readers to understand how to make it concurrent.

Thanks @bhuman, asyncio.gather() was the correct way to go...

I had been doing [other experiments](https://github.com/cclauss/asyncio_hacks/blob/master/factorial_futures.py) to get these asynchronous execution ideas clear in my head.