Implementation for issue-22: XML Test Results Protocol

Implementation for issue #22 

This PR implements the new XML test results protocol as defined in the discussion of issue #22 .

Changes:
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* Change TestResult model to match XML spec
* Change XML parsing in the AutomatedTestsHelper#process_result
* Change student UI to utilize new TestResult model properly

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NOTE1: The changes to the student UI are incomplete (i.e. they do not use TestResult attributes description and feedback). The reason for this is because the student UI is currently being updated on a separate branch. After this branch is merged, these changes will be merged into said branch.

NOTE2: All test scripts must change the XML they output to match the new spec (defined in issue #22).

NOTE3: This PR includes a database migration. After the merge you must run:

    bundle exec rake db:migrate