New - API: plugin(), plugin.register() and plugin.deregister() methods

- When working with plug-ins such as TableTools, there is no clearly
  defined way at the moment to get at the plug-in instance, with each
  doing it its own way (TableTools as a static function, while Scroller
  attaches itself to the settings object and KeyTable is its own
  intialiser, amoung others...) so I'm introducing these methods to
  unify this.

- plugin() is used by DataTables users to get their plugin instance, for
  example `table.plugin('tabletools')` would get the TableTools
  instance allowing full access to its API.

- plugin.register() and plugin.deregister() are called by the plug-ins
  when they attach themselves to a DataTable. The 'extras' will all need
  to be updated to use this new method.