phpdoc

InvalidArgumentException doesn't need to be documented; this is a logic
error (as opposed to runtime error).
[mms] getAvatarUrl() now returns a Horde_Url object.
[mms] Change second parameter to getAvatarUrl() and fetchAvatar() to an array so that additional options can be provided.
[mms] Add 'default' and 'rating' options to getAvatarUrl().