Use 16x16 icon in admin menu

The 32px icon that's being included now does not look right in the admin menu:
![screen shot 2013-09-25 at 4 42 34 pm](https://f.cloud.github.com/assets/1437322/1214437/9b28acb4-263e-11e3-9d79-f68bb5211120.png)