Add rough draft of chapter 8

A couple of notes on this chapter: 

- I've switched chapters 7 & 8. It made more sense to me to explain the app manifest and setting permissions for WebAPIs before we actually get into using the APIs.

- The code examples (using the notification/vibration APIs) reference High Fidelity and rely on the app's ability to download podcast episodes for offline listening. The download feature is apparently still buggy in actual devices, and is currently hidden in the production version. Assuming this gets fixed, I think the code examples are easy to follow and provide some good concepts for new developers to grasp. If the timing of launching our docs vs. download feature works out that these are irrelevant examples, we can switch them around. It's just nice to be able to reference a single application in production for users to follow along with.

- We'll want to cleanup the code in hifi to check for a complete download. =P It is sloppy at the moment. (#L63-69).