make menu to support sub-menu; some api docs update
Merge branch 'master' into polymer-element

Conflicts:
	polymer-ui-menu-item/polymer-ui-menu-item.html
	polymer-ui-menu/polymer-ui-menu.html
	polymer-ui-nav-arrow/polymer-ui-nav-arrow.html
	polymer-ui-sidebar-menu/polymer-ui-sidebar-menu.html
Merge branch 'polymer-element' of github.com:Polymer/polymer-ui-elements into polymer-element