Adds API endpoint listing all operator shelves upon which the current user has permission to operate (bug 1074368).

![Mine.](https://cloud.githubusercontent.com/assets/23885/4565352/25abe82e-4f1f-11e4-866f-e56ec9b14b8a.gif)

r? @ngokevin @robhudson @diox 
> not sure how REST-ful this URL is

This one isn't very RESTful, but it follows the pattern set in [other API endpoints](http://firefox-marketplace-api.readthedocs.org/en/latest/topics/accounts.html#account).

> Can't it be part of subfeedshelf ? If it can't, I'd prefer for the line to be moved next to the other /feed/shelves/ lines for consistency.

It can't, unfortunately, without making it its own ViewSet, which seemed overkill. And can't be moved down either, or else it gets [pegged as a slug](https://github.com/mozilla/zamboni/pull/2641/files#diff-b5ae258622dea3015012cc5a7a4ce981R46).