auto merge of #798 : alexcrichton/cargo/issue-777, r=brson

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

diff --cc src/cargo/core/manifest.rs
Simple merge

diff --cc src/cargo/sources/registry.rs
Simple merge

diff --cc tests/test_cargo_publish.rs
Simple merge