add createImage, setImageURL

![screen shot 2014-12-15 at 2 25 53 pm](https://cloud.githubusercontent.com/assets/5429146/5445236/f007bc0c-8466-11e4-9436-e1685902797d.png)

* add createImage and setImageURL methods for webapp
* modified setText, getText to set/get the alt text on image elements