A+C: Kai Backman (individual CLA)

Just AUTHORS; already in CONTRIBUTORS from work at Google.

R=gri
CC=golang-dev
https://golang.org/cl/2098048

AUTHORS

@@ -57,6 +57,7 @@ Joe Poirier <jdpoirier@gmail.com>
 Jonathan Wills <runningwild@gmail.com>
 Josh Goebel <dreamer3@gmail.com>
 Jukka-Pekka Kekkonen <karatepekka@gmail.com>
+Kai Backman <kaib@golang.org>
 Kei Son <hey.calmdown@gmail.com>
 Ken Friedenbach <kenliz@cruzio.com>
 Kevin Ballard <kevin@sb.org>