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 
I'm following the precedent @ngokevin set in other feed item-type factories, e.g. https://github.com/mozilla/zamboni/pull/2641/files#diff-1c376e0be387c2122cf2ab62198c6a57L66

I don't think it's bad. It's a factory for tests, sensible defaults are probably fine.