PECropView: Allow to programmatically set the rotation angle (with or without snapping to 90° angles)

This methods allow to easily implement clockwise/counter clockwise rotations from IBAction methods.
```objective-c
- (IBAction)rotationButtonTapped:(id)sender
{
    [_cropView setRotationAngle:_cropView.rotationAngle + M_PI_2
                           snap:YES];
}
```