Issue 463: Provide a signout\logout method

Src/GoogleApis.Auth.WP/OAuth2/GoogleWebAuthorizationBroker.cs

Index: Src/GoogleApis.Auth.WP/OAuth2/GoogleWebAuthorizationBroker.cs
===================================================================
--- a/Src/GoogleApis.Auth.WP/OAuth2/GoogleWebAuthorizationBroker.cs
 b/Src/GoogleApis.Auth.WP/OAuth2/GoogleWebAuthorizationBroker.cs
@@ -75,6 75,23 @@
 return await AuthorizeAsyncCore(initializer, scopes, user, taskCancellationToken).ConfigureAwait(false);
 }
 /// <summary>
 /// Asynchronously reauthorizes the user. This method should be called if the users want to authorize after
 /// they revoked the token.
 /// </summary>
 /// <param name="userCredential">The current user credential. Its <see cref="UserCredential.Token"/> will be
 /// updated. </param>
 /// <param name="taskCancellationToken">Cancellation token to cancel an operation.</param>
 public static async Task ReauthorizeAsync(UserCredential userCredential,
 CancellationToken taskCancellationToken)
 {
 var installedApp = new AuthorizationCodeWPInstalledApp(userCredential.Flow);
 // Create an authorization code installed app instance and authorize the user.
 UserCredential newUserCredential = await installedApp.AuthorizeAsync(userCredential.UderId,
 taskCancellationToken).ConfigureAwait(false);
 userCredential.Token = newUserCredential.Token;
 }
 /// <summary>The core logic for asynchronously authorizing the specified user.</summary>
 /// <param name="initializer">The authorization code initializer.</param>
 /// <param name="scopes">
@@ -83,13 100,14 @@
 /// <param name="user">The user to authenticate.</param>
 /// <param name="taskCancellationToken">Cancellation token to cancel an operation.</param>
 /// <returns>User credential.</returns>
- private static async Task<UserCredential> AuthorizeAsyncCore(AuthorizationCodeFlow.Initializer initializer,
- IEnumerable<string> scopes, string user, CancellationToken taskCancellationToken)
 private static async Task<UserCredential> AuthorizeAsyncCore(
 GoogleAuthorizationCodeFlow.Initializer initializer, IEnumerable<string> scopes, string user,
 CancellationToken taskCancellationToken)
 {
 initializer.Scopes = scopes;
 initializer.DataStore = new StorageDataStore();
- var installedApp = new AuthorizationCodeWPInstalledApp(initializer);
 var installedApp = new AuthorizationCodeWPInstalledApp(new GoogleAuthorizationCodeFlow(initializer));
 return await installedApp.AuthorizeAsync(user, taskCancellationToken).ConfigureAwait(false);
 }
 }