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 now calls external rubric parser to make testing easier
* basic unit tests for xml parsing confirm things seem to work
* max available points calculated at rubric ingestion time now
Merge pull request #33 from edx/jrbl/126-peer_criteria_explanations

TIM-126: Peer criteria support explanations