Update and stabilize SemVer

http://discuss.rust-lang.org/t/stabilizing-semver/376/18 got a lot of discussion, but kinda died out.

What do you think?
Oh sorry! The changes haven't been merged yet. They're on my `cargo-upload` branch:

https://github.com/alexcrichton/cargo/blob/cargo-upload/src/cargo/core/version_req.rs#L83-L91