Provide quickcheck_macros as a Cargo-able crate.

Cargo now has support for metapackages (essentially searching repo for
multiple Cargo.toml), so now one can use `#[quickcheck]` by including

    [dependencies.quickcheck]
    git = "https://github.com/BurntSushi/quickcheck"
    [dependencies.quickcheck_macros]
    git = "https://github.com/BurntSushi/quickcheck"

in your `Cargo.toml`.
Update README for our new Cargo overlords.