Annotated tags

Waiting on this to get merged which adds the feature to gitlab-shell: https://github.com/gitlabhq/gitlab-shell/pull/162

Looking for feedback.  Willing to change things if necessary.  Thanks!
@jvanbaarsen I don't disagree but the msg wasn't part of the JSON response, and the tests failed after I added it.  It's been a couple weeks but I remember looking into this and stopping once I realized I would also have to update the `gitlab_git` gem to support finding the `msg` for annotated tags.  Unfortunately, I'm leaving for a week long vacation right now.  I was hoping to get this merged and then circle back when I could update that gem.