Adds performance report tasks

Code is directly form [PagerDuty/pd-cap-recipes](https://github.com/PagerDuty/pd-cap-recipes/blob/master/lib/pd-cap-recipes/tasks/performance.rb)

[Also worth reading for speeding up asset pipelines](http://blog.pagerduty.com/2012/10/growing-a-rails-application-how-we-made-deploy-fast-again/)