Add onSave for saveJson

Hello.

For a profesionnal project, I had to use callbacks after saving with saveJson plugin. 
As I didn't found out how to make that, I add onSave parameter to this plugin. 

It's possible to use it like this : 
```
$('#element-to-edit').raptor({
    plugins: {
        // The save UI plugin/button
        save: {
            // Specifies the UI to call the saveJson plugin to do the actual saving
            plugin: 'saveJson'
        },
        saveJson: {
            // The URI to send the content to
            url: 'save.php',
            // The POST parameter name to use (e.g. for $_POST['raptor-content'] in PHP)
            postName: 'raptor-content',
            // The ID of the content (or a function the returns the ID)
            id: function() {
                return this.raptor.getElement().data('id');
            },
            // Run this callback after saving is done
            onSave: function(data, status, xhr) {
                console.log(data);
                console.log(status);
                console.log(xhr);
            }
        }
    }
});
```

I'm not sure it's the best way to do that, I'm really new to raptor. 

Have a nice day !
And thanks for your wonderful editor.