Update the UI around using crates.io

Try to purge the word "registry" as much as possible in favor of just referring
to crates.io as "crates.io". This also enables the default index as being the
official rust-lang crates.io index.
auto merge of #844 : alexcrichton/cargo/less-registry, r=brson

Try to purge the word "registry" as much as possible in favor of just referring
to crates.io as "crates.io". This also enables the default index as being the
official rust-lang crates.io index.