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 
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.