Issue 463: Provide a signout\logout method

Src/GoogleApis.Auth/OAuth2/Flows/IAuthorizationCodeFlow.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 50 matching lines @@
 CancellationToken taskCancellationToken);
 
 /// <summary>Asynchronously refreshes an access token using a refresh to
ken.</summary>
 /// <param name="userId">User identifier.</param>
 /// <param name="refreshToken">Refresh token which is used to get a new
access token.</param>
 /// <param name="taskCancellationToken">Cancellation token to cancel ope
ration.</param>
 /// <returns>Token response which contains the access token and the inpu
t refresh token.</returns>
 Task<TokenResponse> RefreshTokenAsync(string userId, string refreshToken
,
 CancellationToken taskCancellationToken);
 
- /// <summary>Asynchronously revokes the specified access token.</summary
 >
+ /// <summary>
+ /// Asynchronously revokes the specified token. This method disconnects 
 the user's account from the OAuth 2.0
+ /// application. It should be called upon removing the user account from
  the site.</summary>
+ /// <remarks>
+ /// If revoking the token succeeds, the user's credential is removed fro
 m the data store and the user MUST
+ /// authorize the application again before the application can access th
 e user's private resources.
+ /// </remarks>
 /// <param name="userId">User identifier.</param>
 /// <param name="token">Access token to be revoked.</param>
 /// <param name="taskCancellationToken">Cancellation token to cancel ope
ration.</param>
 /// <returns><c>true</c> if the token was revoked successfully.</returns
>········
 Task RevokeTokenAsync(string userId, string token, CancellationToken tas
kCancellationToken);
 }
 }