Add v1.0.0-beta spec.

Copied from https://raw.github.com/mojombo/semver.org/a16c419f09b5c5a26320317fcd331264daae16ef/index.md. Ref issue #52.
Label it as a beta.
Link to 1.0.0-beta.
Merge pull request #56 from theory/1.0.0-beta

1.0.0 beta