Add gravatar directive

Source: https://github.com/lavinjj/angularjs-gravatardirective
License: MIT
WIP: Comments tab on DBE

A few features remain, but what's here already is working.

Working:
 - Gravatars by email address of comment owner
 - "Post New Comment" button
 - Comments persist in data model

Not yet implemented:
 - Score buttons change score
    + Score buttons should also record who clicked them
 - Subcomments
 - Collapsable comments section (collapse link might show "N comments")
 - Comment status can be changed (selected from dropdown)
 - "Post New Comment" field hides, only shows itself when "Reply" clicked