Support for partial reveal using subviews and sample project fix

This adds the ability to assign a subview via cell.viewToReveal which is animated instead of cell.contentView. This is useful for revealing only a portion of a large cell, such as a button bar, while leaving the rest of the cell intact. By default self.viewToReveal is set to self.contentView to preserve current functionality. It also sets the next reveal direction when cell.direction is assigned so that if the first or only reveal is a programmatic one via cell.revealing = YES, it will use the intended direction instead of always starting from the left. Finally the sample project has a compile error from backView removal that's been updated to use backgroundView so everything runs happily in the simulator again.