Button Alpha Change Fix

Changing the alpha value of the button removes the vibrancy effect.
Adding a custom property "alpha" and setting the overlay alpha values with respect to this value fixes this problem.

Normal State:
![normal](https://cloud.githubusercontent.com/assets/4599500/3735924/12e6798a-1729-11e4-9973-d853e83a558d.jpg)

Changed Alpha:
![alphachange](https://cloud.githubusercontent.com/assets/4599500/3735880/a3255e7c-1728-11e4-81d1-432992534cb1.jpg)

Changed Alpha with Fix:
![alphafix](https://cloud.githubusercontent.com/assets/4599500/3735905/ee489e5a-1728-11e4-845d-555b5bf87478.jpg)
Please add an empty line here :)