Explained the use of config.url in the README and added contextual resolution of image src attributes

It was unclear how to control what url jsdom uses to resolve anchor href attributes, so I clarified in the README.

I also found that it would be useful if jsdom treated image src attributes the same way (I was manually resolving the relative 'src's of all my images), so I added that functionality. If this was not included yet intentionally, maybe it should be configurable. But if the default for anchors is to automatically resolve 'href's, I figured by analogy this would be the correct default.