Added get_css_classes to the Content model.

The previous implementation of class_to_json wasn't calling css_classes
because you can't call property methods without an instance.  I added a
method get_css_classes which works on both instance and classes.  This
allows widget creators to set css_classes on their widgets and for that
to work in both to_json and class_to_json.
Button css_classes have changed.
Merge pull request #203 from fusionbox/buttonWidget

Change Button's css_classes so that it doesn't confict so hard.