0.0.3

Built off of rustup.sh, includes rustup.sh in the installation.
multirust update, without specifying a toolchain, will update all three of 'stable', 'beta' and 'nightly'.
Added multirust run <toolchain>, which configures the environment of a subprocess so that multirust will use a specific toolchain.
Removed update notifications. Just use multirust update, which will be a no-op if there's no new revision to install.
multirust now can validate hashes with either sha256sum or shasum.
Added multirust upgrade-data for upgrading metadata.
Updated metadata format from version 1 to 2.

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