Add wrapper <span> for checkbox style-ability

Because a checkbox is a replaced element it's not possible (in all
browsers) to use ::before/::after to generate content based on the
:checked pseudo-class (which is useful if you want to replace the
checkbox image).  The simplest workaround is to have an element
following the checkbox which can be targeted with the + selector.
Merge pull request #25 from twm/stylish-checkboxes

Add wrapper <span> for checkbox style-ability