$.formParams(): Return changed element/set nested field names

This PR comes in two parts:

a) $.formParams() should support returning only the changed element:

``` 
$('form').change(function(event){
    console.log( $(event.target).formParams() ) ;
});
```

b) $.formParams() should be able to set nested field names:

```
// Sets <input name="foo[bar][baz]" value="Hello World">
$('form').formParams({
	foo: {
		bar: {
			baz: "Hello World"
		}
	}
});
```