Created uuid command to generate new UUIDs.

In Drupal 8 we will be dealing with UUIDs, see for example [Add UUIDs to default configuration](https://drupal.org/node/1969800). Here is a simple drush command that will output a fresh UUID on the command line.

    $ drush uuid
    81779421-39c3-422e-8500-d1e7854ce8df