Fixed issue with exception when IMAP transport returned no messages.

--HG--
branch : ashvetsov/fixes-to-push
extra : source : b9b283ff46d734933c4a715630e114d5e250a34b
Fixed issue that made it possible for signal receiver to break `getmail`.

--HG--
branch : ashvetsov/fixes-to-push
extra : source : a76f8a4e1cf9c469710eb30d093be749965ee213
Merged in medsafe/django-mailbox/ashvetsov/fixes-to-push (pull request #1)

Thanks @medsafe!

Fixes for signal exception handling and inability to get IMAP inbox
Added tag 3.0.2 for changeset 60b47cbeda21
Bumping version number.