[FEATURE] Track call start/end time and duration

Inbound calls are timed from their offer until end event. Outbound calls are timed from answered to end events. Duration is available live and frozen on end.

#318
[FEATURE] Add post-join duration to Call's metadata when created by #dial

#318
[FEATURE] Add per-call result to DialStatus

#318
[CS] Stray useless comment
Merge pull request #339 from adhearsion/feature/dial_extra_status

Extra #dial status