DRY default retransmission
Helpers for UAS mode

* send_trying (alias send_100)
* send_ringing (alias send_180)
* send_answer
* receive_ack
* answer (send_answer + receive_ack)
Fix error in comment
DRY unused variable handling

This prevents SIPp from blowing up if we set a variable but later don’t reference it anywhere
Notify users when errors occur in scenario compilation
Typo
Example incoming call scenario
Support for sending a hangup on an existing call
WIP
Spec fixes
Rework scenarios to use correct addresses and tags
Documentation update
Merge pull request #72 from mojolingo/feature/uas_helpers

Feature: UAS helpers