Fixed menu label displacement, if it's the first child.

If menu contains a label that is the first child, by default it has `.5` margin to the left, causing a slight offset from the center.

This fixes that.

Test & Example here: http://jsfiddle.net/psycketom/mAAn5/

Encountered this while using labels as unread notification counter within the menu.

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Just an idea: maybe this should be changed globally (for labels), and added a feature where label could also be on the left handside (of vertical menus ... haven't currently experienced any more uses of labels)?

That'd need, at least, added a right margin, and, possibly an extra rule for `.label:only-child`.

@jlukic, what do you think about left-aligned labels?