New model to track examiner provided comments

The comments in this table are overlaid - not embossed - on a student's scan
Merge pull request #134 from adamarla/master

Distribute scans to allow for non-admin examiners
[Big one]: Replaced canvas based grading with overlay-based one
Merge branch 'master' of heroku.com:drona
Merge branch 'preview'
Removed orange border around overlay
Corrected associations to address run time errors
 Removed redundant alert
[Bug]: Only the first TeX comment being over-laid

Must move overlay.counter forward even when adding a non-event comment.
The counter goes into defining the <script> id and MathJax engine needs
it to be different to render different TeX comments
[grading]: Mark the last graded response as graded and highlight it as such
Added new method to clear a preview
[Bug]: Clear preview as part of feedback detachment

Otherwise images from the detached preview are carried over
and this causes carousel controls to barf
[Bug]: fdb.attach / detach interplay

Carousel for loading preview solutions hangs every now and then.
Still haven't been able to ascertain the exact conditions under
which this happens or why it happens. Seems to happen, however, after 2-3 toggles

Sharpened some of fdb.method definitions. Helped a bit. But
perhaps we need different carousels for scans and previews
Enable / disable keyboard if auditing during grading ...

Disable when starting the audit, re-enable when audit submitted
New action to completely reset a previously graded response ...

Clicking on restore will not only reset the feedback, but will destroy any associated comments
Merge remote-tracking branch 'upstream/master'