upgrade to tornado 3

ships minimal subset of tornado in zmq.eventloop.minitornado

If tornado 3 is present, the zmq IOLoop is a minimal subclass of tornado's own IOLoop,
in the same model as the EPoll / KQueue subclasses in tornado itself.

I believe the API used by tornado 2.x is unchanged, so old apps shouldn't have any issues,
but I haven't tested enough to be certain.