Guide testing

The start of a testing guide. This PR relies on the crates and modules one because I shuffled some stuff around, so sorry about that.

I got stuck with how to import this name with cargo. @wycats and @alexcrichton , any ideas?
Integration tests are meant to test a library or the commands produced by a package. You'll likely want a `src/lib.rs` which you can `extern crate foo` here.