Added an avatar method to the User class.
We use the Gravatar service to provide user avatars. The method returns the user's image scaled to the requested pixels.
An md5 hash of the email is created and incorporated into the URL. After that a number of options are available to customize the avatar.
'd=mm' determines the placeholder image incase the user has no Gravatar. mm refers to the mystery man image. 's=N' requests the avatar to scale to given size.