The avatar services registry is a new object for managing avatar

services, which are defined to provide multiple ways of displaying

users' avatars. Currently, the only supported service is the Gravatar

(https://gravatar.com) service.
Avatars support the srcset attribute for <img> tags. In browsers

where it is not supported, a polyfill is used.

Ran unit tests
Used this with an upcoming patch in Review Board.
Tested the srcset polyfill in IE11 with Review Board.