Made various changes to support the following:

  role of 'main' around main sections of document
  role of 'complementary' around appendices
  role of 'navigation' on table of contents section
  role of 'note' on notes
  role of 'definition' on definitions
  aria-describedby on dfn elements linking to their definitions
  aria-describedby and aria-label on references to internal definitions
  aria-labelledby on definitions pointing to their associated dfn
  elements
  Added tests

  (closes #304)