This change creates the DBus signals `SyncMessageReceivedV2` and `MessageReceivedV2`, as well as the new data type `DbusAttachment`.
A DbusAttachment is a struct that contains considerably more information than the filename.
To maintain backward compatibility, `SyncMessageReceived` and `MessageReceived` remain unchanged.
V2 includes Mentions and DbusAttachments.
This change also permits sending arbitrary URLs as attachment names.
create new DbusAttachment type
allow URLs for --attachment option
update manpage
update wiki with signalmail
implement setExpirationTimer() for DBus
implement isRegistered() for DBus
add sendNoteToSelfMessageWithDBusAttachments
add sendGroupMessageWithDBusAttachments
add sendMessageWithDBusAttachments
bump version
