Added validations for radio buttons groups

And in the correct branch this time! :D

Anyways, I refactored the changes a little to perform some extra checks and remove some stupidity.  Namely, I added checks for the case where you have a uniquely named radio button and want to validate it.  The group checks now only work when you have more than one radio button with the same name.

There's still no way to make a validation like "is this radio button in this group of buttons (not) selected", but I still think that those cases are either rare or incorrectly using radio buttons.

I also felt pretty stupid after I found the checks.  Right at the top of the "checker.coffee" file. ..