[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
[FEATURE] SIP INFO DTMF
Remove unused packets attribute
[FEATURE] Don't write unnecessary PCAP files

Now that DTMF can be sent using SIP INFO, it is likely that scenarios
will be created without media. SIPp can be built without PCAP support
so having sippy_cup effectively require it unnecessarily is not very
helpful, especially given that recent Ruby versions have trouble
writing the media packets. This also removes the nasty {{PCAP}} path
hack.