Clarify the documentation on `cargo test`

Closes #812
Don't look at custom build targets for output locs

They don't have any! We also care more about the other targets regardless.

Closes #821
Don't panic on unused overrides

This previously did a failing hashtable lookup when an `Option`-returning `get`
would suffice.
auto merge of #816 : alexcrichton/cargo/issue-812, r=brson

Closes #812
Cache build output in the target build directory

Placing it in the host directory may end up later on down the road causing a
spurious recompilation when one isn't necessary. I couldn't currently think of a
test case for this, as I don't think that this affects correctness.
auto merge of #828 : alexcrichton/cargo/issue-821, r=brson

They don't have any! We also care more about the other targets regardless.

Closes #821
Tweak doc style to match crates.io
auto merge of #829 : alexcrichton/cargo/less-paniks, r=brson

Unused overrides previously led to a panic, and build cmd outputs were cached in the *host* location instead of the *target* location.
auto merge of #840 : alexcrichton/cargo/new-docs, r=brson

Rendered version: http://alexcrichton.com/cargo/

Closes https://github.com/rust-lang/crates.io/issues/14
Upgrade all deps to using build scripts