add OrderedList, UnorderedList, and ListItem elements
add structural elements
sort attributes for the benefit of tests
add remove (and remove_raw) to ChildGenerator and HTMLChildGenerator
add class NonVoidElement as base class for Element

As opposed to the Element class, NonVoidElement does not have any child handling.
add (HTML)ChildGenerator.children
fix two tests
add module htmlgen.document

Contains classes: Document, HTMLRoot, Head, Body, Title, Meta, Script, HeadLink, Main