Add drush command for running casper tests (integrate https://github.com/Lullabot/casperjs_foundation)

I'd like to bring in the work done on https://github.com/Lullabot/casperjs_foundation into the drupal foundation. It seemed to me to make more sense to rewrite testrun as a drush command as well, to take advantage of automatically knowing what URL to use with drush aliases.