Allow dump_tree to take a textual document
Make note of another interesting test case for lists
Implement nested lists
Move commonalities between Document and Element into parent role
Use last_child instead of children->[-1]
Add more diag info on test failure
Add test for mixing ordered/unordered lists
Automatically chomp text docs for test_doc
Test an unordered sublist under an ordered list
Add test for interleaving paragraphs and lists
Make sure lists end the current paragraph
Add names for list tests
Put nested sublists at the same level as list items
Allow _extract_attributes to include private attrs
Put ListElement objects directly under parent lists

It was the case that nested list objects were placed under the preceding
ListItemElement in the structure:

  Document
    ListElement
      ListItemElement
      ListItemElement
        ListElement
         ListItemElement
      ListItemElement

This is probably how the resulting HTML will be structured, but this is
potentially specific to HTML output, and not really indicative of the
document's structure.  The HTML emitter will have to deal with this
special case in the structure, but that's a small price to pay.