Add a new 'divider' field

The 'divider' field has the following custom attribute:
- header: heading text of the divider. Optional.
Screenshot: http://i.imgur.com/3bC7hyC.png
Add 'button' field
split divider field into two separate fields: 'header','divider'
Merge pull request #315 from pryley/master

New fields: divider/header/button