Optimise PNG with pngquant

Thanks @paperboyo for mentioning that we did not optimise pngs to the extent allowed by pngquant.

Just as an example, here's the example of savings we make on sprite.png:

### Sprite.png: before (38 KB)

![sprite-before](https://cloud.githubusercontent.com/assets/85783/3527268/3b09bf40-0783-11e4-826c-831d5f07feb6.png)

### Sprite.png: after (17 KB)

![sprite-after](https://cloud.githubusercontent.com/assets/85783/3527274/41a84c04-0783-11e4-861c-e72c29229f24.png)

![ ](http://gifs.joelglovier.com/lolwat/werkin-out.gif)