Support significant_terms in terms panel

[fvlankvelt]

With Elasticsearch providing the 'significant_terms' aggregation, it would be great to be able to use that in Kibana. Such a query provides valuable insights to users, revealing the 'uncommonly common' terms in subsets of the data.

The elasticjs client master supports building aggregations with the fluent api, making the implementation quite easy to do. (after upgrading - see #1222)

[spalger]

Hey @fvlankvelt Kibana 4 supports the Significant Terms aggregation, and while it doesn't provide configuration options for all of it's settings we have #1527 planned for Beta 2. Stay tuned!