CONTRIBUTORS: add Dave Cheney <dave@cheney.net>
CONTRIBUTORS: add Dave Cheney <dave@cheney.net>

R=golang-dev
CC=golang-dev
http://codereview.appspot.com/4257065

polydawn-ports/goprotobuf,ruiaylin/goprotobuf,andrewzeneski/goprotobuf,t3rm1n4l/goprotobuf,tonnerre/go-protobuf,go-mirrors/goprotobuf,VividCortex/goprotobuf,wfxiang08/goprotobuf,adwinsky/goprotobuf,liumuqi/goprotobuf,supiyun/goprotobuf