support shard_size for facet requests

[mschoch]

When issuing facet requests through an alias, you can get incorrect results. Using the "shard size" feature as described in this Elasticsearch page can help:

http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-facets-terms-facet.html

[mschoch]

Moving this to 1.x since it would be an additive change to the API it could be added later.