Adding active support instrumentation

The code change adds Active support instrumentations to AWS cloudsearch calls.

Cloudsearchable clients can subscribe to these instrumented calls using active support notification subscriber. For example to subscribe to search calls, client application should create a subscriber like this:
ActiveSupport::Notifications.subscribe('cloudsearchable.execute_query') do |*args|
  events << ActiveSupport::Notifications::Event.new(*args)
end