close #39

Now xcoder_b2b has 3 different modes for dealing with Max-Forwards
header.
close #38
close #40

Resolvido memory leak no módulo b2b_logic.
Fix #41

Access to opensips sdp internal structures are now more protected.
Fix #42 - Refer-To header is duplicated

Refer-To header is duplicated and Max-Forwards is missing on transfered
calls
Merge remote-tracking branch 'origin/master' into 1.3