Added classes 'checkbox-group' and 'radio-group' to allow clean vertical formatting of checkbox and radio input types

Allows a group of checkboxes or radio buttons to be aligned vertically.  An example from the new output:

![screen shot 2014-06-28 at 11 58 51 pm](https://cloud.githubusercontent.com/assets/6226188/3422711/0a29be5e-ff5c-11e3-8651-a667219b362c.png)

Once a div with class "checkbox-group" or "radio-group" is created, any number of checkbox or radio combos can be wrapped inside it.  The 'input' tag MUST be wrapped inside of the 'label' opening and closing tags, unlike the horizontal example in the docs, which as the 'input' tag outside of the 'label' element.

In all of my use cases, this has worked wonderfully in any grid setting and screen size.