Auto merge of #1373 - alexcrichton:issue-785, r=wycats

Auto merge of #1373 - alexcrichton:issue-785, r=wycats

This commit now funnels all output of Cargo by default to be in `target/debug`
instead of the bare `target` directory. This change is targeted at raising
awareness of whether a debug build is being used (as opposed to a release
build). It is also aimed at remedying a common scenario where `cargo build` is
followed by `cargo build --release` and then the debug binaries are run by
accident.

This does not yet explore the option of providing symlinks to the most recent
build, hence this commit is a breaking change due to the restructuring of the
layout of the output.

Note that this commit does **not** change the output location for documentation.
All output of `cargo doc` continues to be funneled into the `target/doc`
directory.

Closes #785