Add setBackgroundFromUrl method

[karlvr]

This allows loading of cross origin images by adding crossOrigin: 'anonymous' to the options object. Using the existing setBackgroundFromDataUrl method it is not possible to use cross origin images as that implementation loads the Image first, and doesn't include a crossOrigin setting.

The same cross origin issue probably exists in addImg but I have not had a chance to investigate yet!

[karlvr]

I've taken the liberty of adding a setBackgroundFromImage method too. This refactors the existing setBackgroundFromDataURL method to load the Image and then use the new method. I've used this form when I load the Image first in order to get things like its width and height, and then it's nice just to pass it directly to react-sketch.