[FEATURE] Rayo events should be timestamped with dispatch or receipt time

If an XEP-0203 (Delayed Delivery) tag is included, we use it's timestamp; otherwise, we use the time of receipt.
[FEATURE] Copy timestamps from AMI events to Rayo events for signalling

Ringing, Answered, Hangup, Join, Unjoin, etc
Merge pull request #213 from adhearsion/feature/timestamped_events

Support timestamped Rayo events