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)

Surrounding space missing for operator '='.