[RAILS] Added ActiveSupport::Instrumentation integration

Require the component in your `application.rb` file:

    require 'elasticsearch/rails/instrumentation'

You should see an output like this in your application log in development environment:

    Article Search (321.3ms) { index: "articles", type: "article", body: { query: ... } }

Also, the total duration of the request to Elasticsearch is displayed in the Rails request breakdown:

    Completed 200 OK in 615ms (Views: 230.9ms | ActiveRecord: 0.0ms | Elasticsearch: 321.3ms)

See:

* http://edgeguides.rubyonrails.org/active_support_instrumentation.html
* http://api.rubyonrails.org/classes/ActiveSupport/Notifications.html