Make ElasticSearch JSONP response optional.

This adds the http.jsonp.enable option, which left enabled by default at the reqeust of the Elasticsearch folks. Allowing JSONP responses by default on all API endpoints poses a security risk, and probably shouldn't be used unless it's necessary.

This also squashes a couple bugs in NettyHttpChannel.java
 - JSONP responses were never setting application/javascript as the content-type
 - The content-type and content-length headers were being overwritten even if they were set