Wrapper around _all() to cache and return all results of something

        >>> ec2.instances.all()
        [ ... ]

def all(cls):
        """
        Wrapper around _all() to cache and return all results of something

        >>> ec2.instances.all()
        [ ... ]
        """
        if not hasattr(cls, '_cache'):
            cls._cache = cls._all()
        return cls._cache