- add slav, zoa, and royk to the CREDITS for the generic jitterbuffer

- change references to the "scx" jitterbuffer to be called "fixed" and change
  references to the "stevek" jitterbuffer to be called "adaptive", instead


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@31356 65c4cc65-6c06-0410-ace0-fbb531ad65f3