API - dev: Remove the plugin() methods from the new API

- Decided that it would be better to provide the new API by having
  plug-ins extend it directly, rather than providing their own API.
  Using the plugin() method it was possible to get a plug-in insta,ce
  for example TableTools, but the way it should work is that TableTools
  would provide a tabletools() API method, registering it as an
  extension to the DataTables API objects. Further more, TableTools
  should provide methods such as row().select() etc, extending the
  current API rather than doing its own thing which the plugin() methods
  prompted.

- No backwards compatiblity issues here as the plugin() code hasn't
  shipped as a release.