Element should override Node methods, such as removeAttr

[tobia]

Methods that are inherited from Node and not overridden, return a Node instance, even when they are called on an Element instance. This prevents chaining:

Element foo = ...
foo.addClass("isfoo").removeAttr("bar"); // OK
foo.removeAttr("bar").addClass("isfoo"); // ERROR Node#addClass not found

[jhy]

Makes sense, would welcome a PR if you want to do one?