Owners endpoint and resource rename to match Pivotal Tracker API

I added the /projects/{project_id}/stories/{story_id}/owners endpoint so I could get the owner names of stories.

I also renamed Me to Person since that appears to be the actual name of the resource used by Pivotal Labs (also a bunch of Mes didn't make sense for Owners).

The formatting is wrong, somehow. Also please add `,omitempty` into the `json` tag everywhere...