Give more real-world examples of asynchronous tasks

[jmoldow]

All of the examples right now are toy examples, such as factorial().

Learners who are interested in asyncio are probably interested in both the "async" and the "io" parts. So give more real-world examples, instead of just toy examples. Even if (in the earlier chapters) it is just pseudo-code.

Do asynchronous file operations, as well as asynchronous network calls. Add more aiohttp examples.