Adds Drush 6 formula for stable beta release

The developers of Drush encouraged the Drupal community at DrupalCon Portland 
to start using Drush 6 (beta release but stable enough for production use).

This is more of a temporary formula until the main Drush formula is
updated to use Drush 6. This formula creates a `drush6` symlink so
it should avoid conflicts with existing Drush installations.