minor improvements

[rubnov]

When setting up a table view cell in a storyboard or xib file, initWithStyle: isn't called and therefore initialize is missed. I've added awakeFromNib: to invoke initialize as an alternative.
In addition, to suppress warnings about required JASwipeCellDelegate methods, I've declared all delegate methods optional. This is a suggestion, of-course. I didn't see a need to make the methods required.