UIView+JDFlipImageView was ignoring flip direction

Thank you for this implementation. It's been a tremendous time saver for me. 

I noticed that the UIView category was ignoring the JDFlipImageViewFlipDirection and always flipping from the top. I only modified the private factory method. The public API remains unchanged.
I realize I accidentally included indentation change in this pull request. I'll fix that if you want me to.