Add option for dataTables callbacks and jQuery plugin

- Callbacks for the dataTables plugin can now be specified in the WidgetUI
  class and any function that uses it
- oxfordCoursesWidget() jQuery wrapper added so that the functionality can
  be attached whenever wanted, and not just to .courses-widget-container
- Currently the options for the plugin just include the callbacks to send
  to dataTables
Document an example assignment of `callbacks` here please