Create annotated tags for releases

[benma]

Hi

It would be great if you tagged the current HEAD with v1.0.0, and in the future, occasionally bump the version and make new tags with new releases. I suggest using the semver.org versioning scheme, where you have major.minor.patch, and you bump the major for incompatible changes and the minor for compatible changes to the API.

[agl]

You may wish to know that I'm promoting this package to x/crypto: https://go-review.googlesource.com/#/c/22030/.

This version, in my personal GitHub, will then be reserved for more experimental stuff. (Although I don't expect to change the core API any.)

[benma]

Good to know, thanks!

If it is gonna end up in x/crypto, please consider merging #16 and #17 (esp. the latter). If there is a permission/licensing issue, feel free to make the changes in your name, or whatever is easiest :)