auto merge of #302 : alexcrichton/cargo/doc-test, r=wycats

Whenever `cargo test` is run and a testable library target is available, the doc
tests will be run. This can be opted out of with `test = false` as usual.

This is currently not super useful due to rust-lang/rust#16157, but I expect
that to be merged soon. In the meantime examples will need to `extern crate foo`
explicitly.

Closes #334

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

diff --cc src/cargo/util/toml.rs
Simple merge