htmldiff: initial ugly integration of htmldiff.js library

This appears to be a better written library and I think a more promising
route moving forward.  There are some parts of the resultant output
that are not desirable right now, but I think some of that can be resolved
by some cleanup and styling changes.

@funkyeah This package is available on npm, but I wasn't sure how to integrate it so I just added it directly (as it was a single file and already written in coffeescript).  It isn't "perfect" but seems to work better -- we may need to customize.  Feedback on the right way to integrate appreciated.