misc/dist: more gophers for OS X installer

Replaces the default OS X installer background
with a more fitting image.

R=golang-dev
CC=golang-dev
https://golang.org/cl/6116043

misc/dist/bindist.go

@@ -256,6 +256,7 @@ func (b *Build) Do() error {
 		targ := base + ".pkg"
 		_, err = b.run("", "productbuild",
 			"--distribution", filepath.Join(dist, "darwin/Distribution"),
+			"--resources", filepath.Join(dist, "darwin/Resources"),
 			"--package-path", pkgdest,
 			targ)
 		if err != nil {

misc/dist/darwin/Distribution

 <?xml version="1.0" encoding="utf-8" standalone="no"?>
 <installer-script minSpecVersion="1.000000">
     <title>Go</title>
+    <background mime-type="image/png" file="bg.png"/>
     <options customize="never" allow-external-scripts="no"/>
     <domains enable_localSystem="true" />
     <installation-check script="installCheck();"/>