code: public static TypeElement getSuperClassElement(Element element) {
  		if (element.getKind() != ElementKind.CLASS) {
  			return null;
  		}
  		TypeMirror superclass = ((TypeElement) element).getSuperclass();
  		if (superclass.getKind() == TypeKind.NONE) {
  			return null;
  		}
  		DeclaredType kind = (DeclaredType) superclass;
  		return (TypeElement) kind.asElement();
  	}
docs: Retrieves the super class of the given {@link Element}.
  Returns null if {@link Element} represents {@link Object}, or something other than {@link ElementKind#CLASS}.
  @param element target {@link Element}.
  @return {@link Element} of its super class.
  @author vvakame