Update to rust master
auto merge of #776 : alexcrichton/cargo/issue-488, r=alexcrichton

This helps us not unwrap() and rather propagate the error upwards.

Closes #488
Move generated tarballs under target/package

At the same time this commit renames the `.tar.gz` extension to `.crate`. This
helps our perception on Windows as we're not trying to leave them out in the
dark, and we'd also like the ability to modify the format later in the future.

Closes #777