Block and monitor multiple streams for new data.
        streams: a dict of stream names to stream IDs, where
                   IDs indicate the last ID already seen.
        count: if set, only return this many items, beginning with the
               earliest available.
        block: number of milliseconds to wait, if nothing already present.

def xread(self, streams, count=None, block=None):
        """
        Block and monitor multiple streams for new data.
        streams: a dict of stream names to stream IDs, where
                   IDs indicate the last ID already seen.
        count: if set, only return this many items, beginning with the
               earliest available.
        block: number of milliseconds to wait, if nothing already present.
        """
        pieces = []
        if block is not None:
            if not isinstance(block, (int, long)) or block < 0:
                raise DataError('XREAD block must be a non-negative integer')
            pieces.append(Token.get_token('BLOCK'))
            pieces.append(str(block))
        if count is not None:
            if not isinstance(count, (int, long)) or count < 1:
                raise DataError('XREAD count must be a positive integer')
            pieces.append(Token.get_token('COUNT'))
            pieces.append(str(count))
        if not isinstance(streams, dict) or len(streams) == 0:
            raise DataError('XREAD streams must be a non empty dict')
        pieces.append(Token.get_token('STREAMS'))
        keys, values = izip(*iteritems(streams))
        pieces.extend(keys)
        pieces.extend(values)
        return self.execute_command('XREAD', *pieces)