Add 16x16 favicon for non-retina screens

This pull requests adds back the original 16x16 favicon for non-retina users. #93 replaced it with a 32x32 icon, but this had the effect of producing a washed out icon for non-retina users.

The ico now includes both the 32x32 and 16x16 icons.

Before and after:
![screen shot 2014-11-17 at 16 53 32](https://cloud.githubusercontent.com/assets/2204224/5085970/27fb184c-6f0f-11e4-9038-3fff20785552.png)

There should be no change for retina users.