TIM-126: Peer criteria support explanations

* Rubric criteria options now support an optional <explain>
  child, which is used as explanatory text for that particular
  criterion option, e.g.:
  5 - Very Well (indicates intelligent use of persuasive
                 argument to, etc etc.)
* Sample explanations added to the Poverty sample rubric to
  show them in use.
* parse_xml refactored to make it easier to see the forest for
  the trees.
* max available points calculated at rubric ingestion time now