initialize
Arguments
first
if an Attribute, clones the attribute. If a String, uses this as the attribute name. If a Source, parses the source for key=value pairs.
second<nil>
(optional) if first is an Attribute or a Source, this can be the Element to which the Attribute belongs. If first is a String, this is the value part of the attribute.
parent<nil>
(optional) if first is a String, this can be the parent to which this attribute belongs
Constructor.