[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] receive_message for incoming SIP MESSAGEs

I did want to abort the call on a regexp mismatch but every method I
tried simply didn't work. stop_call is only effective before the call
is answered and who knows how the <jump> action works.

ack_bye has been renamed to okay (and aliased) because nothing about
it is specific to BYE.
Clarify #receive_message test for variable declaration and add test for variable name incrementing.
Changelog entry