Updated text and image to show that QS supports Mavericks

Changed the text and added/updated image with support of Mavericks.
Also changed the paths to be correctly cased.

This commit still has the v1.1.0 in the `download.php`, as `QSApp.com/master` hasn't been updated either.

I'm actually not sure what the status of QS on Mavericks is, so if it's not yet supported,
feel free to at least use the updated images for Mountain Lion + Lion.

[lion mountainlion mavericks@2x.png](https://f.cloud.github.com/assets/112204/1577560/9a65b840-516c-11e3-81cd-66733853851a.png)
[lion mountainlion mavericks.png](https://f.cloud.github.com/assets/112204/1577561/9aa2b8da-516c-11e3-800d-8f5d84f8d432.png)

[lion-mountainlion@2x.png](https://f.cloud.github.com/assets/112204/1577549/75c862da-516c-11e3-909d-040eec1984d5.png)
[lion-mountainlion.png](https://f.cloud.github.com/assets/112204/1577550/7604647e-516c-11e3-9727-03920ce5d3e1.png)

(I'm not extremely fond of the combined usage of old-style ML/Lion-discs and the new flat Mavericks-icon, but I don't have the time and skills to make it better)