"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 
This still does not look like it will support an out-of-tree build: 'src/doc/trpl' will not contain the source for rustbook in that case. [These comments](https://github.com/rust-lang/rust/pull/19897#discussion_r22015799) are still relevant. If that is implemented then the three additions to the .gitignore file are unnecessary. Out of tree build can be tested like

```
mkdir build && cd build && ../configure && make docs
```