better whiteboard icon

I cleaned up the whiteboard icons in photoshop and set colors / transparency to the Holo Light iconography rules. Holo Dark versions can easily be rendered from the photoshop file which is included; I can't be bothered right now as I don't use night mode.

![untitled](https://cloud.githubusercontent.com/assets/2818274/4140002/476c8fee-33a0-11e4-87d1-41503c72ef99.png)