Support the "field stats" API

The [field stats](https://www.elastic.co/guide/en/elasticsearch/reference/1.7/search-field-stats.html) API is very handy for picking out min / max values for fields along with other metrics. We should add this to the client.

/cc @grantr @chrismwendt

Start by reading the linked Elasticsearch field stats API documentation and inspecting the client’s existing API entry points and tests. Add client support for retrieving field statistics such as minimum and maximum values, then verify the supported metrics through focused tests; the issue does not name specific files.

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