Shorter plugin initialization and reset/clear form through data attributes

With this pull-request you'll have 2 new small features:

- Shorter plugin initialization. This feature works for both ajaxForm and ajaxSubmit.
```
$( '#form' ).ajaxForm( url, data, dataType, function(response){
   //success callback function
});
```

- data attributes to set resetForm/clearForm

```
<form action="" method="POST" data-reset="true"></form>
<form action="" method="POST" data-clear="true"></form>
```