Generate AUTHORS file with a script

This information is in git, so it's better to use that as a source than
updating this file manually. This script can be run manually at release
time for now, or we can set up a cron job sometime in the future.

Change-Id: I0344135ceb9c04ed14e2e2d939a93194e35973db

tools/gen_authors.sh

+#!/bin/bash
+
+# Add organization names manually.
+
+cat <<EOF
+# This file is automatically generated from the git commit history
+# by tools/gen_authors.sh.
+
+$(git log --pretty=format:"%aN <%aE>" | sort | uniq)
+Google Inc.
+The Mozilla Foundation
+The Xiph.Org Foundation
+EOF