Update to iOS SDK 3.20, fix uses of /me/permissions

* Fix uses of /me/permissions that depend on pre v2.0 behavior.
* Update to iOS SDK version 3.20.
When looking the permissions to request you can perhaps simplify all this to:

    [permissionsNeeded removeObjectsInArray: currentPermissions];

Also you'd pass in permissionsNeeded instead of requestPermissions to the API request. Lastly, you'd change permissionsNeeded to be a NSMutableArray, ex:

    NSMutableArray *permissionsNeeded = [@[@"public_profile", @"user_birthday"] mutableCopy];