Add auto-upload functionality to site logo form.

#4421


Before upload:
![autoupload-before](https://f.cloud.github.com/assets/3273122/1563261/b786ff12-505c-11e3-8717-da7a9419e465.png)


After upload:
![autoupload-after](https://f.cloud.github.com/assets/3273122/1563262/b9751822-505c-11e3-8ef8-047562a1f521.png)