[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