Implement testing/demo mode

I need to demo this to my client in real so I can't wait for the next scheduled delivery. A "send now" button, or a "send on next cron run" checkbox, on admin/settings/simplenews/digest_settings would be helpful.

For now one needs to perform these tests in order to demo:

1. set up admin/settings/simplenews/digest_settings as you normally would
2. create some new nodes
3. add the following lines just before the call to _simplenews_digest_cron_routine($current_time, $start_time, $end_time); in the simplenews_digest_cron() function.

  $start_time = time() -86400;
  $end_time = time() - 3600;

4. run cron

Cheers,

Albert.