Issue 463: Provide a signout\logout method

Src/GoogleApis.Auth/OAuth2/AuthorizationCodeInstalledApp.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 48 matching lines @@
 
 public async Task<UserCredential> AuthorizeAsync(string userId, Cancella
tionToken taskCancellationToken)
 {
 // Try to load a token from the data store.
 var token = await Flow.LoadTokenAsync(userId, taskCancellationToken)
.ConfigureAwait(false);
 
 // If the stored token is null or it doesn't have a refresh token an
d the access token is expired we need·
 // to retrieve a new authorization code.
 if (token == null || (token.RefreshToken == null && token.IsExpired(
flow.Clock)))
 {
- // Create a authorization code request.
+ // Create an authorization code request.
 var redirectUri = CodeReceiver.RedirectUri;
 AuthorizationCodeRequestUrl codeRequest = Flow.CreateAuthorizati
onCodeRequest(redirectUri);
 
 // Receive the code.
 var response = await CodeReceiver.ReceiveCodeAsync(codeRequest,
taskCancellationToken)
 .ConfigureAwait(false);
 
 if (string.IsNullOrEmpty(response.Code))
 {
 var errorResponse = new TokenErrorResponse(response);
 Logger.Info("Received an error. The response is: {0}", error
Response);
 throw new TokenResponseException(errorResponse);
 }
 
 Logger.Debug("Received \"{0}\" code", response.Code);
 
 // Get the token based on the code.
 token = await Flow.ExchangeCodeForTokenAsync(userId, response.Co
de, CodeReceiver.RedirectUri,
 taskCancellationToken).ConfigureAwait(false);
 }
 
 return new UserCredential(flow, userId, token);
 }
 
 #endregion
 }
 }