json encoding python types

[adq]

Hi, one obvious problem is the lack of support for encoding custom types... for example datetime.datetime currently fails saying its not JSON serializable.

The best way to do this would be to pass an encoder class to the json.dumps() call in send_request. I was thinking if you could supply an encoder and decoder class when you call the ESClient constructor it would give the maximum flexibility, but still keep the ESClient API simple.