Better str representation of HyperShuffleProducer


![screenshot 2014-06-20 22 36 43](https://cloud.githubusercontent.com/assets/720571/3347949/ca5501e0-f906-11e3-85e7-a20949ebd865.png)
Change the str representation of `HyperShuffleProducer` according to the discussion today.
@dhalperi