Added Button Style Fill

I added another Button Style "AYVibrantButtonStyleFill" to get a Button that uses a overlay with "AYVibrantButtonOverlayStyleInvert" as default and adjusts it's alpha on touch
![normal](https://cloud.githubusercontent.com/assets/4599500/3734671/366f08ea-171a-11e4-9b41-49f9020c0d02.jpg)
![active](https://cloud.githubusercontent.com/assets/4599500/3734672/36717c92-171a-11e4-8464-d44a82d50daf.jpg)