uber: Add Uber API endpoints

The endpoints are documented here:
https://developer.uber.com/docs/rides/authentication#oauth-20

Change-Id: I5fc0fff1f48393938548bc5071bc35f32d259a7a
Reviewed-on: https://go-review.googlesource.com/26690
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

uber/uber.go

@@ -0,0 +1,16 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package uber provides constants for using OAuth2 to access Uber.
+package uber // import "golang.org/x/oauth2/uber"
+
+import (
+	"golang.org/x/oauth2"
+)
+
+// Endpoint is Uber's OAuth 2.0 endpoint.
+var Endpoint = oauth2.Endpoint{
+	AuthURL:  "https://login.uber.com/oauth/v2/authorize",
+	TokenURL: "https://login.uber.com/oauth/v2/token",
+}