Add support for reindex

[gmarz]

https://github.com/elastic/elasticsearch/blob/master/rest-api-spec/src/main/resources/rest-api-spec/api/reindex.json

For the 2.x branch we'll need to back port this without breaking the existing "client" reindex implementation.

[Mpdreamz]

merged to 2.x and master as per #2017