"The Rust Programming Language"

This is a preliminary PR that significantly changes Rust's documentation.

TL;DR: Remove the distinction between "the guide" and "the guides", and merge them into one big 'ol "The Rust Programming Language." You can see it rendered here: http://steveklabnik.github.io/the_rust_programming_language/

This uses https://github.com/aturon/rust-book to generate said final product.

Basically, the new "book" as I'm calling it looks like this:

Part I: Basics <- this is the first half of the current Guide. Linear.
Part II: Intermediate <- this is some of the current Guide, with the invididual Guides replacing chapters where relevant. Can be read in any order.
Part III: Adavanced <- the most advanced of the previous Guides. Can be read in any order.

Conceivably, this could even fix https://github.com/japaric/rust-by-example/issues/278 by making Rust by example a part IV or something. While Rust By Example doesn't currently use Rustbook, it could.

I don't think the `Makefile` is currently up to snuff, but it works, at least. Ish. Feedback very welcome.

This also adds an optional dependency on rustbook, which may not be something that we want.