Auto-construct all o-ft-headers in body on o.DOMContentLoaded event.
Add --js data attribute to header root element on construction of header.
o.DOMContentLoaded event listener is now removed once it's been called.
Documented event-based initialisation.
Removed redundant bubble property.
Merge pull request #28 from Financial-Times/auto-construct

Auto-construct all o-ft-headers in body on o.DOMContentLoaded event.