Add visited styles for all tones

Using the `mix($color1, $color2, percentage)` because `:visited` doesn't allow for opacity changes, so the new value needs to be pure hex, not rgba.

![screen recording 2014-10-17 at 05 19 pm](https://cloud.githubusercontent.com/assets/1607666/4682116/62895a32-5619-11e4-8ac3-de5c31b74239.gif)