Initial structure for generic inband dtmf detection.  NOTE: This code does not currently remove the inband dtmf tones from the stream (as that is not supported using read media bugs) and it is also COMPLETELY untested and unused anywhere.  This code is there for somone who wants to implement this feature in mod_sofia to test.

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@4111 d0543943-73ff-0310-b7d9-9358b9ac24b2