Client radiobuttongroup

Radiobuttons can now use validations.

This will become useful when the Submit-button starts validating the whole form (which doesn't work yet).
Until then, you can test this functionality by setting `matches[otherfield]` as validation rule and see that that validation works.

This also contains preliminary work to get select and checkbox working (they might even work but I didn't test).

I used one model+view for every radiobutton. This might seem excessive, but this will make it easier to add radio-buttons and select-options using a template on the fly. I also seems slightly cleaner.