oauth2/google: add config type to use Cloud SDK credentials

Change-Id: Ied7fecc0cb155c33faca7766b81221eacb3aa0c0
Reviewed-on: https://go-review.googlesource.com/1670
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Burcu Dogan <jbd@google.com>

google/example_test.go

@@ -74,6 +74,19 @@ func ExampleJWTConfigFromJSON() {
 	client.Get("...")
 }
 
+func ExampleSDKConfig() {
+	// The credentials will be obtained from the first account that
+	// has been authorized with `gcloud auth login`.
+	conf, err := google.NewSDKConfig("")
+	if err != nil {
+		log.Fatal(err)
+	}
+	// Initiate an http.Client. The following GET request will be
+	// authorized and authenticated on the behalf of the SDK user.
+	client := conf.Client(oauth2.NoContext)
+	client.Get("...")
+}
+
 func Example_serviceAccount() {
 	// Your credentials should be obtained from the Google
 	// Developer Console (https://console.developers.google.com).