Initial commit for creating plugin documentation into asciidoc

Fixes #1741
Add Plugin docs in ascii format

Fixes #1741
[DOCS] Add index page generation for each plugin type

Fixes #1741
Minor doc fixes

Fixes #1741
Fix for default codec. Based on PR #1829

Fixes #1741
Add HTTP Auth and SSL to the ES output plugin

The typical use case is the placement of a transparent proxy in front of
ES. This PR enables basic Auth and HTTPS to be configured independently.

This patch makes logstash use only elasticsearch-ruby gem when writing
to elasticsearch. Also leverages the manticore http transport that ships
with es-ruby for the basic http auth and ssl features.