HTMLForm mutators can now be chained

HTMLForm mutators can now be chained

This make HTMLForm mutator to return $this for easy method chaining such
as:

	form = new HTMLForm( $someFields );
	$form->setMethod( 'get' )
		 ->setWrapperLegendMsg( 'message-key' )
		 ->suppressReset()
		 ->prepareForm()
		 ->displayForm();

prepareForm()->displayForm(); should be at the very end apparently
though it is not enforced by the object.

Change-Id: Iebaa83a1da3c89e5ab729a889e54844535619d54