Cookbook update

Builds on top of #84, editing some descriptions, but mostly fixing old stuff.

From the commit message:

* Rewords the async() description
* Replaces both async() examples with one actual practical example
* simplifies the keylogger example
* updates links to external sites
* removes further tutorials, since they're all outdated
* removes book reference, will put in footer
If that's rendering ok, there's nothing else to complain on this commit, it's looking good.