Support for ABPadButton hightlightedTextColor

Add support for `[[ABPadButton appearance] setHightlightedTextColor:]` and transition `numberLabel` / `lettersLabel` during `touchesBegan:` and `touchesEnded:`.

By default, `highlightedTextColor` is the same colour as `textColor`, for backwards compatibility due to the transitions.

This allows for more customization, like making `textColor` and `selectedColor` the same colour, and still be readable when tapped.

![abpadlockscreen-highlight](https://f.cloud.github.com/assets/71955/2522522/338e795e-b4c2-11e3-9dee-07623361d051.png)