Commenting on an error when there is only one user raises an exception

As described in issue #452.

``ArgumentError: At least one recipient (To, Cc or Bcc) is required to send a message``

It looks like this was introduced in commit: 310b4fc

When the comment author is the only notification_recipient, there are no recipients for the email.

This pull request fixes that by putting the logic of when to send an email notification in the ``Comment#emailable?`` method (like the one in App). A comment is "emailable" when its app is emailable and has recipients other than the comment author.