Update favicon to include 32x32 image for retina screens

### Chrome (vs current)
![chrome](https://cloud.githubusercontent.com/assets/203886/4392639/9ac33fe6-4410-11e4-90d2-3aff0c431775.png)

### Firefox (vs current)
![firefox](https://cloud.githubusercontent.com/assets/203886/4392642/9dd20d2a-4410-11e4-9565-932c5846c81b.png)

### Safari
![safari](https://cloud.githubusercontent.com/assets/203886/4392657/bf39e6d6-4410-11e4-81eb-555b6fe60b74.png)

### IE6
![ie6](https://cloud.githubusercontent.com/assets/203886/4392658/c3cb97d0-4410-11e4-9dd1-5e2dceba0fba.png)

### IE8
![ie8](https://cloud.githubusercontent.com/assets/203886/4392661/ca0b3844-4410-11e4-9dde-44a873e6b10c.png)

### IE10
![ie10](https://cloud.githubusercontent.com/assets/203886/4392662/cf483582-4410-11e4-80c8-996dce8460ba.png)

### IE11
![ie11](https://cloud.githubusercontent.com/assets/203886/4392667/d4cffb8e-4410-11e4-90d2-de0f2b97e9ad.png)