NSURL *url = [NSURL URLWithString:@"http://example.com/"];
AFOAuth2Client *oauthClient = [AFOAuth2Client clientWithBaseURL:url clientID:kClientID secret:kClientSecret];

[oauthClient authenticateUsingOAuthWithPath:@"/oauth/token"
                                   username:@"username"
                                   password:@"password"
                                    success:^(AFOAuthCredential *credential) {
                                        NSLog(@"I have a token! %@", credential.accessToken);
                                        [AFOAuthCredential storeCredential:credential withIdentifier:oauthClient.serviceProviderIdentifier];
                                    }
                                    failure:^(NSError *error) {
                                        NSLog(@"Error: %@", error);
                                    }];