Added autosize.resized event + tests

Useful if you want to know when the height has changed also w/out knowing where/how the autosave plugin has been initialized. The `autosize.resized` event will be triggered when height changes. 

### how to use it

```JS
var myTa = $('#textareaID');
// init autosize
myTa.autosize();
myTa.on('autosize.resized', function(){
   console.log('on autosize.resized');
});
```

Added also some simple tests (see the `test` folder)