Returns a link to the gravatar of someone based on his email address, see the Gravatar website if you don’t know what this is about.
email: email address of the user for whom you want the gravatar
size: the size in pixels of the served image, defaults to 80
default: an url to the default image to display, or one of the three image generators: identicon, monsterid or wavatar, see http://en.gravatar.com/site/implement/url for more infos on those, by default this will be the gravatar logo
rating: the highest allowed rating for the images, it defaults to ‘g’ (general, the lowest/safest) and other allowed values (in order) are ‘pg’ (parental guidance), ‘r’ (restricted) and ‘x’ (boobies, crackwhores, etc.)