added Material Design color palette for panels

Hi,
First of all, thanks for your project!
I've started to use it just recently, and I've noticed that Material Design color palette was not working with panels.
So I've just added it (I used color palette from here: http://www.google.com/design/spec/style/color.html#color-ui-color-palette ). It works the same way as everything else (btn, etc).
I'm a beginner, this is my first pull request so I hope I didn't do anything wrong. You can add it to your project if you like it.