Add optional close button

Added a close button which is hidden by default. To show it, set the hidden property of `closeButton` to `NO` 
``` objc
- (void)showFrenchman {
     ESImageViewController *ivc = [[ESImageViewController alloc] init];
     ivc.closeButton.hidden = NO;
     [ivc setTappedThumbnail:self.extraThumbnail];
     [ivc setImage:self.extraThumbnail.image];
     [self presentViewController:ivc animated:YES completion:nil];
```