Allow radio buttons to be "de-selected"

[pferreir]

It's impossible to select "None" in a radiobutton field that is not mandatory. It's true that a dropdown menu would be the best solution for such situation, but truth is that it's possible to do it and it makes sense (at least IMHO), e.g. by "de-selecting" a radio button once the user clicks on it.
We have, however, to be careful from the UX point of view. The behavior should be clear and predictable.