Adds the following helpers to example groups with `:type => :mailbox`

* process(mail_or_attributes) - send mail directly to the mailbox under test for
  `process`ing.
* receive_inbound_email(mail_or_attributes) - matcher for asserting whether incoming
  email would route to the mailbox under test.
* have_been_delivered - matcher for asserting whether an incoming email object was delivered.
* have_bounced - matcher for asserting whether an incoming email object has bounced.
* have_failed - matcher for asserting whether an incoming email object has failed.

Also adds an ActionMailbox test generator