Merge pull request #114 from edx/dev

Dev
Merge pull request #115 from edx/dev

Dev
Merge pull request #116 from edx/dev

Dev
Merge pull request #117 from edx/dev

Handle the case where rubric scores are not filled out if answer unknown...
Merge pull request #118 from edx/dev

Add louder rubric parsing errors
ammended 'Not Enough Calibration Essays' message to be more friendly and descriptive
Merge pull request #119 from edx/giulio/changed-message

ammended 'Not Enough Calibration Essays' message to be more friendly and descriptive
Fixed typo in usage documentation
Merge pull request #120 from edx/will/docs-typo

Fixed typo in usage documentation
Fix peer grading required count