Merge pull request #722 from shirosaki/enigmail

Fix pgp signing failure with Enigmail 1.5.1 on quick reply
Properly display messages even if the sender's email address is empty.
Fix an error to display pgp message with Enigmail 1.5.1

enigmailSvc.mimeInitialized() was removed.
http://sourceforge.net/p/enigmail/source/ci/0a50a6d763b46761e283fc01ba1dd766ab2ddd88