oauth2, oauth2/jwt: break JWT off into its own package

Change-Id: Iaaa36728f87744e0d9609674f0d0ad96e6ac80b4
Reviewed-on: https://go-review.googlesource.com/2198
Reviewed-by: Burcu Dogan <jbd@google.com>

google/example_test.go

@@ -15,6 +15,7 @@ import (
 
 	"golang.org/x/oauth2"
 	"golang.org/x/oauth2/google"
+	"golang.org/x/oauth2/jwt"
 	"google.golang.org/appengine"
 	"google.golang.org/appengine/urlfetch"
 )
@@ -76,7 +77,7 @@ func ExampleJWTConfigFromJSON() {
 func Example_serviceAccount() {
 	// Your credentials should be obtained from the Google
 	// Developer Console (https://console.developers.google.com).
-	conf := &oauth2.JWTConfig{
+	conf := &jwt.Config{
 		Email: "xxx@developer.gserviceaccount.com",
 		// The contents of your RSA private key or your PEM file
 		// that contains a private key.