Add sense = [AsRecipient | AsSender]

and

party = [the username of the other end]

to GET /transmits with backwards compatibility for senderUserName
(senderUserName is equalent to sense=AsRecipient&party=senderUserName)
Default for sense is AsRecipient.

Also changed AudioResource to permit fetch for sender or receipent to
allow the sender to playback audio they sent.