Support for horizontal picker that doesn't auto-select items in the middle nor animate items to the middle

In order to support the ability to have an interactable horizontal picker that DOES NOT auto-select items in the middle, we've added a shouldCenterSelectedItem property that defaults to YES.  However, if you set this to NO, two things will happen:

1. The picker won't automatically scroll the selected item to the center. As a side effect, the horizontal picker no longer has padding on the left and right to allow for the first or last item to be scrolled to the center. The picker's content extend only as far as the first and last element.

2. Since there isn't an obvious selection point anymore, you don't automatically select whichever item is in the center.  To select an item requires clicking on the item itself.
Adding a horizontal border to the bottom of selected label items

Using the selectionIndicatorView doesn't seem to work for horizontal picker when used with auto layout, since the frame size of the picker is 0x0.  Rather than work through the details of supporting auto layout properly, we just added an optional bottom border width property which is applied to selected labels using the same color as the selected label text.