Implement cargo-package

This command will assemble the current package into a tarball ready for
uploading to the cargo registry. Currently no further verification is done
beyond packaging the local repository into a tarball, but in the future this
could execute other operations such as api stability tools.
auto merge of #423 : alexcrichton/cargo/cargo-package, r=brson

This command will assemble the current package into a tarball ready for
uploading to the cargo registry. Currently no further verification is done
beyond packaging the local repository into a tarball, but in the future this
could execute other operations such as api stability tools.