Fix not working tintColor

Hi, this will fix the not working setTintColor on the DKBackBarButtonItem.

Example code:
 DKBackBarButtonItem * item = [[DKBackBarButtonItem alloc] initWithTitle:@"Menü" style:UIBarButtonItemStylePlain target:self action:@selector(didPressButton:)];
    [item setTintColor:[UIColor blackColor]];
    self.navigationItem.leftBarButtonItem = item;