Add option to show album art in the dock

This is a feature I miss from Vox.

![screen shot 2014-12-12 at 1 03 15 am](https://cloud.githubusercontent.com/assets/2041996/5407716/bae4cee4-819a-11e4-83da-25a032905929.png)
![screen shot 2014-12-12 at 1 03 30 am](https://cloud.githubusercontent.com/assets/2041996/5407718/be67b9c8-819a-11e4-80cc-384f89ccd6eb.png)