Finish up work on annotated tags

- 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/4359059/4398576/1b2fe114-4459-11e4-9c94-954ddd62c17b.png)