Added Autoscaler guide to advanced heroku deployment

This was the proposed change to add an optional step for including and configuring the Autoscaler Gem in the HEROKU.md doc. I've included a pastable version of the Sidekiq initializer file that only adds the autoscaler middleware in production.   
Merge pull request #595 from nverba/patch-1

Added Autoscaler guide to advanced heroku deployment