Squashed commit of the following:
commit 5bcbac33d6
Author: Adam Coddington <me@adamcoddington.net>
Date: Tue Jul 7 23:05:15 2015 -0700
Add handling for situations in which the message was deleted in another thread before processing.
commit 6183f68b39
Author: Adam Coddington <me@adamcoddington.net>
Date: Tue Jul 7 22:58:15 2015 -0700
Always store message as attachment if its content-disposition is marked as such.
commit 4c16494b1a
Author: Adam Coddington <me@adamcoddington.net>
Date: Tue Jul 7 22:46:52 2015 -0700
Adding a (hopefully) failing test case for #52.
