update semver to 0.1.0
move to new interface
Remove version_req.rs in favor of SemVer.

This removes all of the stuff that was in Cargo but should have been in
SemVer, as far as I know.  :confetti-ball:
auto merge of #622 : steveklabnik/cargo/update_semver, r=alexcrichton

This closes the loop on my work of getting semver related stuff out of cargo.

This is also built on top of my other PR, so it's not really ready until that actually merges. But it can stand for review now :)