New: Provide a tabletools() API method for DataTables 1.10's new API

- To get the TableTools instance for a table, you can now use the
  DataTables API method tabletools() which TableTools registers when it
  loads. If a table doesn't have TableTools, you get null back.

- This replaces the `plugin.register()` method which was removed from
  DataTables 1.10's development without shipping in preference of this
  method to encorage good use of the new API.

- TableTools 3 will make much more extensive use of the DataTables API
  with methods such as rows().select() etc, but for the moment, the only
  method is tabletools() to get access to the instance and then use its
  own API methods.