Add option to default Civimail "dedupe e-mail" setting to true

This is to fulfill a request made on the forums here: http://forum.civicrm.org/index.php?action=post;topic=31592

I'm not sure if there's a good use case for this setting defaulting to false, as is currently the case - but in the interests of not upsetting existing workflows, that's what I'll leave it as.