added the ability to clear out an image in the image field type

Sometimes when an image is not required you want to clear it out. The only thing changed in the php was adding a new check for a "cleared" value. Everything else is JS and CSS.

![Screen Shot 2013-02-25 at 5 12 38 PM](https://f.cloud.github.com/assets/1425304/196541/66c1f5f4-801f-11e2-8a7e-03e0db94ae98.png)