Finish up work on annotated tags

After screwing up the first attempt at this PR, here it is again.

- Fixed up JSON response for tags, now includes the `message` and changed API doc to reflect the change.
- Re-added spec for annotated tag creation with the API. Can now verify the `message`.
- Added tag's `message` to tag list. I think the style used for the `message` can be improved, maybe it's not desirable to be displayed at all. I'm hoping for a suggestion here:

![tag_list_with_message](https://cloud.githubusercontent.com/assets/4359059/4398798/beb13a6e-445d-11e4-868f-47fdf0580a44.png)

@Razer6 I think the RepoObject entity was shared by Tags and Branches.  That's the only way it makes sense to me.  When I created the RepoTag entity I dropped it.  According to the git blame, this is the commit that added it: https://github.com/gitlabhq/gitlabhq/commit/2c7554e897356fe424f292c66cd03e0192b05167.  If it should be added back, let me know.