Native checkboxes

This won't work with current less. Also, you probably nested span inside input by mistake?

Me and @gregorpraprotnik agreed that nesting inside label would be better than having to specify three elements in correct order. I'll look into [handlebars](http://handlebarsjs.com/) to see if we can generate correct HTML structure at compile time. But still, we need to agree on the API.

Now I added a support for non-nested labels as well. Current non-nested version works with adding a class to label.

```html
<label for="cb" class="reform-checkbox non-nested reform pure">Not nested label</label>
<input id="cb" type="checkbox" />
<span></span>
```