[BUGFIX] Don't look up scenario node every time
Merge pull request #57 from chewi/bugfix/scenario_node

[BUGFIX] Don't look up scenario node every time
[BUGFIX] Fix username madness

Also, you can't put @ in the source or destination so remove instances
of this from the test data.
Merge pull request #60 from chewi/bugfix/usernames

[BUGFIX] Fix username madness
[FEATURE] Split receive_200 into its own method

It was an alias of receive_answer but receive_answer also stores the
record set and the response time duration, which you probably don't
want to do on every 200 received. Also simplify wait_for_answer and
fix the documentation, which stated that these expectations are
optional by default when they are not.
Merge pull request #61 from chewi/feature/receive_200

[FEATURE] Split receive_200 into its own method
Pull request reference in CHANGELOG
[BUGFIX] Fix ACK/BYE being sent to self

We shouldn't over-rely on the "last" headers provided by SIPp because
if a 200 OK was sent immediately before then the to/from headers will
be reversed.