New: Add callback function to the ajax.url().load() and ajax.reload()
methods
New - API: Static `on()` method, which listens for static events from DataTables

- Some plug-ins would benefit from be able to automatically initialise
  when a DataTables is constructed, to this end, a construct for static
  events is added here, with the static function $.fn.dataTable.on()
  listening for events. Currently only the `construct` method is
  available, although others could potentially be added in future if
  they are useful. As such, the code driving it is intentionally simple
  for this cas,e but the API abstract enough to allow future expansion.

- There is no `off()` method, as I'm not sure it would be that useful.
  Could be added in future if needed!