Sammy.FormBuilder checkbox ( keypath, value, attributes )
creates a checkbox input for keypath with the value
checkboxes can be created with different value, if
value equals the
current value of the key of the form builder's object the attribute
checked='checked' will be added.
checkbox() also generates a hidden element whose value is
the inverse of the value given. This is known hack to get around a common
gotcha where browsers and jQuery itself does not include 'unchecked'
elements in the list of submittable inputs. This ensures that a value
should always be passed to Sammy and hence the server. You can disable
the creation of the hidden element by setting the