More mature error handling around API calls

* Catches SubmissionErrors and sends pass/fail information back to user
  via javascript hidden div.
* Renamed everything from _compose* to just _*
* CSS and js cleanups to make ongoing work less painful (I hope)
Adding a few tests for the Open Assessment XBlock
Merge pull request #21 from edx/jrbl/submit-error-checking

More mature error handling around API calls