Make images responsive.

Images in the QuickLook window aren't adjusting to the width of the window like shown in the image below.

![screen shot 2014-03-18 at 15 28 18](https://f.cloud.github.com/assets/233988/2448760/319e3580-aeaa-11e3-989b-c4a45b62ceca.png)

This commit changes this behaviour, so that images will now adjust to the window size instead.
![screen shot 2014-03-18 at 15 28 39](https://f.cloud.github.com/assets/233988/2448761/31b216ae-aeaa-11e3-9688-a65e8574da94.png)