Add batmanjs favicon

Created a favicon for the batmanjs site; using the logo in purple with a transparent background. This icon has support for retina as well. Let me know what you think!

![screen shot 2013-10-23 at 7 51 19 pm](https://f.cloud.github.com/assets/522911/1395572/19cfaa66-3c3f-11e3-98be-f1fa7b63d13d.png)