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)

You're right, I forgot about this.  I didn't know how to derive the path to the test repo to set the identity there.  Perhaps this is the wrong place to set the identity and it should be done elsewhere?  Any thoughts?