Group.append() replaces Svg.addGroup()

Svg only handles the top group and let the Group class handles the rest
of the elements.
Use introspection to register class with attr 'tag' into svgClass dict.
When a tag is found, an object corresponding to its associated class is
created.
Every SVG object has an 'id'

The object id is the SVG element 'id' or if it does not exists the Python
object id().
Add JSONEncoder class: export SVG group tree to JSON

This is mostly for debugging purposes