Add API v2 URLs (bug 937246), create FeedItem model and API endpoints (bug 937345)

Bugs addressed:

- https://bugzilla.mozilla.org/show_bug.cgi?id=937246
- https://bugzilla.mozilla.org/show_bug.cgi?id=937345

r? @diox @robhudson @spasovski @washort 

Note: this does not include filtering by `region`, `category`, and `carrier`. That's sufficiently complex to warrant another bug (which I will file shortly) and pull request.