Implement `protocol => http` in elasticsearch output

Also add `action` setting to allow logstash to signal document
deletions, updates, etc. The goal is to support any bulk request with
this output plugin, but the early target is simply to allow deletes.

This is one step in the path to having only one elasticsearch output
plugin and eventually removing elasticsearch_http.

The following tests pass
    % bin/logstash rspec spec/outputs/elasticsearch.rb
    ...
    Finished in 1 minute 33.94 seconds
    3 examples, 0 failures

Tested manually 'elasticsearch { protocol => http }' with success.

Still to be implemented:
* template management
* tests to cover all protocols: node, transport, and http
* mark elasticsearch_http as deprecated?