[STORE] Added a Rails ORM generator for Elasticsearch::Persistence::Model

Usage:

    $  bundle exec rails generate scaffold Person name:String email:String --orm=elasticsearch --force
[STORE] Added the template for generating a full Rails application with persistence model

Usage:

    rails new music --force --skip --skip-bundle --skip-active-record --template /path/to/template.rb
    rails new music --force --skip --skip-bundle --skip-active-record --template https://raw.githubusercontent.com/elasticsearch/elasticsearch-rails/persistence-model/elasticsearch-persistence/examples/music/template.rb