[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
Merge pull request #58 from chewi/feature/receive_message

[FEATURE] receive_message for incoming SIP MESSAGEs
[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.
Merge pull request #56 from chewi/bugfix/last_headers

[BUGFIX] Fix ACK/BYE being sent to self
[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.