made some delegate methods optional

- (void)AVCaptureStarted:(DIYAV *)av;
- (void)AVCaptureStopped:(DIYAV *)av;
- (void)AVDidFail:(DIYAV *)av withError:(NSError *)error;
- (void)AVModeWillChange:(DIYAV *)av mode:(DIYAVMode)mode;
- (void)AVModeDidChange:(DIYAV *)av mode:(DIYAVMode)mode;

are now optional.