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.

Benefits improved from casperjs_foundation:
* Automatically setting the --url parameter based on the current drush alias, rather than hard-coding it in the tests or test runner.
* Rather than having a cookie file in the test running directory, uses drush temporary file function to give us a random file for cookies in each test run.
* Supports setting a test-root option in the project's drushrc.php file (example provided in commented-out code in the boilerplate drushrc.php).