New: ajax API methods:

- Introducing several methods which will control the ajax aspects of
  DataTables through the API:

  - ajax.json() - get the last JSON returned from the server
  - ajax.reload() - reload from JSON source
  - ajax.url( [url] ) - get / set the Ajax URL
  - ajax.url( url ).load() - load data from new URL after a set

- Note that this effectively replaces the old fnReloadAjax plug-in which
  was quite popular.

- Not yet fully tested - further work required.