IOS - Extract permissions from session object instead of retrieving them with ...

In IOS plugin, permissions should be extracted from session object instead of being retrieved  with a call to /me/permissions.

There are 2 problems with current implementation.

* A unnecessary call is done to /me/permissions is done in place of retrieving permissions from activeSession object
* The NSDictionary *currentPermissions does not have permission value as keys so the current implementation always fails to check permissions.