Use drush runserver to provide an env for testing, validation

Relates to issue #39.

This PR includes a `drush:liteinstall` and `drush:runserver` task, which performs a Drupal site install using the standard install profile to a SQLite database, and uses PHP's internal web server to host the installed Drupal site. This allows Behat tests to be quickly run against the site install.

This PR extends the Travis test to perform these steps and run example Behat tests. This moves the Travis test toward an end-to-end test for Grunt Drupal Tasks functionality.

This branch includes code from the PRs #52 and #53.