Added an activate callback event

Added an 'activate' option which can be used to pass a callback
function. The current tab element and the event are passed to the
callback function.
Added new option to the README and demo HTML file
Added an option to start the tabs closed

The option 'closed' has been added. With this option, the tabs can be
closed on initialisation. You can use the value 'accordion' or 'tabs'
to start closed in there respective views.
Merge pull request #9 from jellekralt/master

Added a tab activation callback