recorder: change title and description

Recorder replaces "%TIMESTAMP" with the start time of the recording.

SIP calls now have the title: "Conversation at %TIMESTAMP
between USER1 and USER2", where the registered username is used if
possible, else falls back on the ring id.

Change-Id: Idd65917173861b395a4bb1476274727679b087b3
Reviewed-by: Sebastien Blin <sebastien.blin@savoirfairelinux.com>