ability to trigger processing from the api

Add 2 method to the api :

``api.processing(true/false)`` and ``isProcessing()``

Example of usage :

```javascript

    $(document).ready(function() {
		    var api = $('#example').DataTable({
			processing:true
                    });
                    
                    console.log(api.isProcessing()); // false

                    api.processing(true);

                    console.log(api.isProcessing()); // true

                    setTimeout(function(){
                                        api.processing(false);
                                        console.log(api.isProcessing()); //false
                    },1500);

```