Add syntax highlighting support for nested block comments.

Swift supports nested block comments so I thought I would go ahead and add that in.

Before my change: 

![screen shot 2014-06-15 at 18 27 20](https://cloud.githubusercontent.com/assets/1159146/3282580/98a37a00-f4f6-11e3-8611-249f6a3aa468.png)

After my change:
![screen shot 2014-06-15 at 18 32 22](https://cloud.githubusercontent.com/assets/1159146/3282582/aa62034c-f4f6-11e3-84ec-511f2cca7923.png)

Pretty cool I think. :grinning: