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 not sure it's the best idea, but perhaps this would be more REST-ful:

```
/api/v2/account/shelves/
```

it would be consistent with the [auth endpoints](http://firefox-marketplace-api.readthedocs.org/en/latest/topics/authentication.html).

but the [accounts endpoints](http://firefox-marketplace-api.readthedocs.org/en/latest/topics/accounts.html) seem to add `/mine/` (e.g., `/api/v2/account/settings/mine/`):

```
/api/v2/account/shelves/mine/
```

so you could do that instead. up to you.

personally, I prefer omitting the `/mine/`. it's super helpful for front-end URLs (e.g., https://addons.mozilla.org/en-US/firefox/user/me/), but I think it's more REST-ful and less awkward to just omit the username to use the authenticated user.

From [GitHub's `Users` API endpoints](https://developer.github.com/v3/users/):

> Many of the resources on the users API provide a shortcut for getting information about the currently authenticated user. If a request URL does not include a :username parameter then the response will be for the logged in user (and you must pass authentication information with your request).

I suppose you don't have to change it here (or change the existing accounts endpoints), but what do you think of this?