Refactor this plugin to be more modular in order to support the java version

[andrewvc]

This plugin needs a major refactor, and will be undergoing one over the next week or so. Keep this in mind if you're submitting a PR! The underlying code will likely all move!

The code in this repository is currently not written in a very modular way. This has created a maintenance nightmare when it comes to sharing code with this plugin and https://github.com/logstash-plugins/logstash-output-elasticsearch_java . Currently fixing a bug means significant back/cross-porting. This process is error-prone and time consuming.

To address this @talevy and I developed the following plan of attack:

1. Move as much code as possible out of elasticsearch.rb and into separate classes/modules
2. After 1. Elasticsearch.rb should be a thin wrapper calling out to those classes
3. Refactor the _java version of this plugin to have this repository as a dependency. Leverage this shared code as much as possible.

[andrewvc]

/cc @Da-wei FYI I just wanted to give you a heads up here! I'll try to integrate #235 into this refactor, but keep this in mind before you start any more PRs! Everything will change (for the better)