Retina favicon.ico

I built a new favicon that includes a 32x32 version to support retina displays.

The 32x32 asset looks like:
![cs-32](https://cloud.githubusercontent.com/assets/220979/3359513/f9dfac9c-fae8-11e3-8cc4-fb360f65ae65.png)

I included the __Existing 16x16__ asset in favicon.ico. However, I noticed that this icon doesn’t match the icon in the CoffeeScript logo exactly. I made a __New 16x16__ asset that’s just a resampled version of the 32x32 version.

|Existing 16x16|New 16x16|
|---|---|
|![existin](https://raw.githubusercontent.com/jashkenas/coffeescript/master/documentation/images/favicon.ico)|![cs-16](https://cloud.githubusercontent.com/assets/220979/3359536/3aa7334e-fae9-11e3-8c7e-5044e402aa8c.png)|

If you want me to update favicon.ico with the __New 16x16__ asset, just let me know. It’s a trivial amount of work.