Updates to API error checking, reporting

* Updates to API error checking and reporting to make it possible to
  report error conditions back to the user (though in production we'll
  generally not want to burden them with this information.)
* CSS and javascript fixes to reflect same.
* Beginning of HTML escaping of prompts introduced, but that part is
  incomplete as yet.
TIM-25: working rubric parsing, display

* Basic parsing of XML rubric's in @ormsbee's proposed format
* Rubrics displayed to user after they've made at least one submission
* Multiple rubrics on the workbench with different evaluation criteria
TIM-25: Responding to PR #30 Feedback

* Removed commented-out nascent Rubric model implementation
Merge pull request #30 from edx/jrbl/xblock_submit_return_type

Jrbl/xblock submit return type