#104 tm module AVP for Reason header

[John Quick]

As discussed in this thread "Re: [OpenSIPS-Users] tm module CANCEL reason on timeout":
Please can a new 'exported parameter' be added to the tm module, that defines an AVP which may be used to allow script interaction to define the Reason Cause Phrase. At present this is hard coded with a "#define" in the timer.c unit. Sometimes, it is useful to be able to modify the reason cause phrase (e.g. to provide cause codes to Q.805 equipment) from the opensips.cfg script. If the AVP is left blank, then it would use the existing reason casue phrase.