Add gravatar icons to list screen

To enable, add gravatar: true to your email field in your model
Reimplement gravatar using underscore method

Added gravatar column mixin.
Email field template forked from Text template, adds image.
Email field underscore method gravatarUrl.
Change event added to public item.js.
Includes Joseph Myers md5.js for client side hashing on change.
Rolled back embedded crypto in the templates.
Updated docs.
Merge pull request #132 from mcauser/gravatar

Add gravatar icons to list screen