Add new key with value.

        Like L{set}, but only stores in memcache if the key doesn't already exist.

        @return: Nonzero on success.
        @rtype: int

def add(self, key, val, time = 0, min_compress_len = 0):
        '''
        Add new key with value.

        Like L{set}, but only stores in memcache if the key doesn't already exist.

        @return: Nonzero on success.
        @rtype: int
        '''
        return self._set("add", key, val, time, min_compress_len)