Rewrite docs for context locals, request contexts, and app contexts

Closes #1151.

The following is an incomplete draft. However, I wanted to request comments immediately, in case anything was severely wrong.

The doc should be updated to include:

- [x] How context-local objects in Flask are implemented

Will squash commits later.
Regarding the second point, docutils actually handles that cleanly.

![](http://i.imgur.com/lEHLpfl.png)