fix focus underline when there's no label

[notwaldorf]

Fixes https://github.com/PolymerElements/paper-dropdown-menu/issues/181

• the original <label> used visibility, not display:none when floating the label. This makes sense for this case too, since display:none-ing the label will also remove the :after, which is the underline
• removed the animation duration if the label doesn't need to float, as this lead to a brief 0.2s of text overlap between the label and the selected text

PTAL @bicknellr, cc @mgiuffrida

[bicknellr]

LGTM; I had no idea that visibility is animatable even though it only has discrete values.

[bicknellr]

http://jsbin.com/ribasiz/edit?html,output