Add INCrement and DECrement functions

(closes issue #15025)
 Reported by: greenfieldtech
 Patches: 
       func_math.c.patch_v4 uploaded by greenfieldtech (license 369)
       slightly modified by me
 Tested by: greenfieldtech, lmadsen


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@198725 65c4cc65-6c06-0410-ace0-fbb531ad65f3