auto merge of #254 : alexcrichton/cargo/cargo-doc, r=wycats

This is blocked until https://github.com/rust-lang/rust/pull/15939 lands, but in the meantime I figured I could get some eyes on this to make sure I'm sane.

Closes #130

diff --cc src/bin/cargo.rs
Simple merge

diff --cc src/cargo/ops/cargo_rustc/context.rs
Simple merge

diff --cc src/cargo/ops/cargo_rustc/fingerprint.rs
Simple merge