Support the new v2 API permissions format.

Facebook v2 API permissions are returned in a different format to v1. See [v1](https://developers.facebook.com/docs/graph-api/reference/v1.0/user/permissions) vs. [v2](https://developers.facebook.com/docs/graph-api/reference/v2.0/user/permissions) documentation.

Instead of:

```
{ "publish_actions": 1, ... }
```

... we now get:

```
[{ "permission": "publish_actions", "status": "granted" }, ...]
```

I was hesitant to dive into adding v2 support in [fb_graph-mock](https://github.com/nov/fb_graph-mock), so the tests are a bit of a hack.
thinking whether this change can break background job cases..