Don't panic on unused overrides

This previously did a failing hashtable lookup when an `Option`-returning `get`
would suffice.
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.
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
Don't count custom compile scripts as targets.
auto merge of #827 : DiamondLovesYou/cargo/master, r=alexcrichton
Upgrade all deps to using build scripts
auto merge of #817 : alexcrichton/cargo/build-cmd, r=brson

This registers a new cargo snapshot as well as updates all dependencies to versions that are using build scripts. This means that overrides can now be used when looking for native libs for cargo!

This also modifies the behavior of `--enable-nightly` to add some extra configuration the buildbot needs to build cargo on CentOS.