Adds immediate yes/no option to iax.conf

This is very similar to the DAHDI immediate=yes option.  When the phone is picked up, instead of giving a dialtone it connects directly to the "s" extension.  Changes where implemented in chan_iax2.c to directly connect to the "s" extension in the appropriate context when this option is enabled.  Examples explaining its use are added to iax2.conf.sample.  CHANGES has been updated as well. 

(closes issue #14266)
Reported by: jcovert
Patches:
      chan_iax2.c.patch-trunk uploaded by jcovert (license 551)
      iax.conf.sample.patch uploaded by jcovert (license 551)
Tested by: jcovert, dvossel
Review: http://reviewboard.digium.com/r/143/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@174046 65c4cc65-6c06-0410-ace0-fbb531ad65f3