Hlayout supports valign (top, middle and bottom)

Currently, the developers have to specify sclass with z-valign-top or similar. It is not consistent with other components (such as Cell, Image...). Thus, it is more consistent and convenient if we allow the user to specify the alignment directly, such as

<hlyaout valign="bottom">
...

See also http://tracker.zkoss.org/browse/ZK-538