Set version based on a git tag of a release rather than cargo.toml

[olevski]

Another suggestion from @Panaetius

It is easy to forget to update cargo.toml.

[eikek]

Who is then updating cargo.toml? Or do we simply not use Cargo.toml for specifying a version?

[Panaetius]

the version field is not required in Cargo.toml (and it defaults to 0.0.0)

[eikek]

Ok, then we simply remove it from Cargo.toml?

[Panaetius]

closed in #62