Improving(?) the way labels and checkboxes are rendered

* Added `'wrapCheckboxInLabel'` option for the ability to wrap solitary checkboxes and label text inside a label, wrapped inside a `div.controls` element
* Made label for groups of radio buttons / checkboxes be rendered as `<div class="control-label">label text</div>`, since labels should only relate to one form element (not a group of them)
* Made `for` attribute on `<label>` only be rendered when form element has an `id` attribute (putting the element's `name` in the `for` attribute isn't valid)
* Moved the automatically-generated hidden `<input>` for solitary checkbox so it's rendered *outside* of the `<label>`