Issue 463: Provide a signout\logout method

Src/GoogleApis.Auth/OAuth2/Flows/AuthorizationCodeFlow.cs

 /*
 Copyright 2013 Google Inc
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
 You may obtain a copy of the License at
 
 http://www.apache.org/licenses/LICENSE-2.0
 
 Unless required by applicable law or agreed to in writing, software
@@ skipping 246 matching lines @@
 // The new token may not contain a refresh token, so set it with the
given refresh token.
 if (token.RefreshToken == null)
 {
 token.RefreshToken = refreshToken;
 }
 
 await StoreTokenAsync(userId, token, taskCancellationToken).Configur
eAwait(false);
 return token;
 }
 
+public virtual Task RevokeTokenAsync(string userId, string token, Cancel
lationToken taskCancellationToken)
+{
+throw new NotImplementedException("The OAuth 2.0 protocol does not s
upport token revocation.");
+}
+
 #endregion
 
 /// <summary>Stores the token in the <see cref="DataStore"/>.</summary>
 /// <param name="userId">User identifier.</param>
 /// <param name="token">Token to store.</param>
 /// <param name="taskCancellationToken">Cancellation token to cancel ope
ration.</param>
 private async Task StoreTokenAsync(string userId, TokenResponse token, C
ancellationToken taskCancellationToken)
 {
 taskCancellationToken.ThrowIfCancellationRequested();
 if (DataStore != null)
@@ skipping 34 matching lines @@
 
 public void Dispose()
 {
 if (HttpClient != null)
 {
 HttpClient.Dispose();
 }
 }
 }
 }