3.0 nest label

I'm opening this PR in an early state so that it can be discussed.

So far, I don't know anyone using cake 3 that has not tried making it work with twitter bootstrap. It is obvious for them that it quite a pain to make the form helper nest inputs inside labels. This makes nesting the default for radio and checkbox widgets.

The approach taken was to let checkboxes render their own label as radio widgets do. The downside of this is that it becomes a pain to do not nest inputs inside labels, and basically impossible to nest other type of inputs inside them. I'm no yet convinced of this, but making a decision before we hit the beta is important.

Once we make the decision I can fix the rest of the tests.