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
adding basic flat HTML templates (with rough cases):

* basic app skeleton HTML
* response HTML
* peer evaluations
* self evaluation
* grade status
* UI messages
Merge pull request #34 from edx/talbs/markup-broadstrokes

Basic/Static Application HTML
Merge branch 'master' into sanchez/template-first-crack

Conflicts:
	apps/openassessment/xblock/openassessmentblock.py
	apps/openassessment/xblock/static/html/t-base.html