Allow html-classes for img

I implemented the feature requested in issue #126. I also updated the [test](https://github.com/trentm/python-markdown2/blob/master/test/tm-cases/html_classes.text) for the html_classes extra.

I'm new to the project, so there might be some things missing. Please notify me if there's a problem with the way I've done this so I can fix them.