Various changes.

Fixes the exist method to check the http status code for parsing instead of looking for an ES response. I wasn't sure how to make this fit into a response so I just made it boolean + err.

Added support for the scan search type which drastically increases the result harvest speed on large result sets in a cluster with many shards.

Added support for calling fields in the response when they have been filtered.

Feel free to cherry pick and/or cut my changes as appropriate. My apologies for clobbering your import lines, I am new to go and couldn't figure out a better way to get everything to work properly.