Update and improve documentation

[natevw]

The documentation could use another pass, especially focusing on:

• General user-friendliness — does it make sense? are the right things emphasized?
• Use of new features (streams, node/xhr options overrides)

In some ways I think the docs still suffer from way too much infatuation with the original "magic" Proxy stuff, to the detriment of showing that — even without the "dot syntax", which I hardly ever use personally! — Fermata is simply a really handy, really clean HTTP library.