Amazon ElasticSearch support

[jchv]

Hi. I am currently using elastigo with Amazon ElasticSearch, and it indeed works. The only problem is, Amazon ElasticSearch is highly limited in how it can be authenticated with; it supports hardcoding IPs, OR using Amazon signed requests. (For those familiar with AWS, it does not even support limiting things via VPC, as Amazon ElasticSearch does not support VPC at all.)

So, I am creating this issue to track the idea of adding support for these signed requests. It would probably be trivial, but to make sure it's done correctly it'd be smart to delegate the actual implementation to another library. Matt Holt created go-aws-auth for this purpose and it would probably be the most minimal way to make this work.

I plan on implementing this in a fork shortly regardless of whether or not it gets merged.

[ukch]

@johnwchadwick What's the current status of your fork? Is it stable? Do you think you could make a PR to merge it back into this repo?