Issue 463: Provide a signout\logout method

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

Index: Src/GoogleApis.Auth/OAuth2/Flows/IAuthorizationCodeFlow.cs
===================================================================
--- a/Src/GoogleApis.Auth/OAuth2/Flows/IAuthorizationCodeFlow.cs
 b/Src/GoogleApis.Auth/OAuth2/Flows/IAuthorizationCodeFlow.cs
@@ -67,5 67,18 @@
 /// <returns>Token response which contains the access token and the input refresh token.</returns>
 Task<TokenResponse> RefreshTokenAsync(string userId, string refreshToken,
 CancellationToken taskCancellationToken);
 /// <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 from the data store and the user MUST
 /// authorize the application again before the application can access the 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 operation.</param>
 /// <returns><c>true</c> if the token was revoked successfully.</returns>
 Task RevokeTokenAsync(string userId, string token, CancellationToken taskCancellationToken);
 }
 }