Fade out and hide the table view cell, instead of changing its background color

This pull request changes the black out effect to fade out effect, as suggested by [@FreddyF](http://www.raywenderlich.com/u/FreddyF), [@ccuenca24@gmail.com](http://www.raywenderlich.com/u/ccuenca24@gmail.com) and others in the [forum](http://www.raywenderlich.com/forums/viewtopic.php?f=20&t=11280). Fade-out gives a smoother animation and has the benefit of showing the background of table view. [Tutorial](http://www.raywenderlich.com/63089/cookbook-moving-table-view-cells-with-a-long-press-gesture) updated accordingly.