Tweak doc style to match crates.io
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.
Fix a panic when building with build scripts

A package can be required to be built for both the host and target architectures
in some cases. For example a crate could be a normal dependency and a build
dependency. Cargo specially handles this case with respect to the build script
to ensure that the build script is run once per output platform.

Cargo also has logic, however, to run the build script only once when the target
and host platforms are the same. In this case Cargo previously wasn't filling in
the local build script output cache for both the host and target platforms, just
the target platform. This commit remedies this situation by ensuring that cargo
populates both the host and target locations in the cache.

Closes #838