Implement Org Team Memberships API

Implements the new [Team Membership API][blog].

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@joeyw @jakeboxer

[blog]: http://git.io/a9jglQ
The endpoint itself returns a JSON dictionary that looks like one of:

```json
{ "status": "pending", "url": "https://..." }
{ "status": "active", "url": "https://..." }
```

or 404s if there's no membership. Is this return value going to work that way?