Button sizes, white button and retina logos

I've added the ability to use "long", "medium", "google", or "short" with the buttonStyle attribute that correspond to the table on https://developers.google.com/+/branding-guidelines. I've added the ability to use the buttonColor attribute with "white" to get the white sign-in button. I've also added support for retina screens by added the @2x logos and media query.

The only thing I'm missing is the hover and focus color for the white button. I couldn't seem to find that hex color anywhere on the brand guidelines site.