terms_stats facet type support

adds terms stats support.

I think it's failing because travis pulling down the wrong version of ES and my code is still based on .9. I can update it if need be. Tests are passing locally for me though.

Much <3 if this can be into .10.
I don't think we need `put_mapping` unless we're incrementally building the index. If you look at other tests, they just wipe the index and add a new mapping or they set the mapping in the class attributes. Either one is better since it exists already.