"The Rust Programming Language," Take II

An updated version of https://github.com/rust-lang/rust/pull/19461

This version vendors aturon/rust-book@731f7bf and builds it when building the docs. This is almost great, except my `make`-foo is poor, so I have my own personal paths in `mk/docs.mk`. How should I best get around that?

/cc @brson 
They end up all going into the final build. This ends up replacing the Crates section of guide.md

> On Dec 18, 2014, at 21:23, Alex Crichton <notifications@github.com> wrote:
> 
> In src/doc/guide-crates.md:
> 
> > @@ -1,569 +0,0 @@
> > -% The Rust Crates and Modules Guide
> These files look like they were all deleted, were they supposed to be moved instead?
> 
> \
> Reply to this email directly or view it on GitHub.
>