Support labels with attribute bracket notation like post[author][name]

It allows to write the following code in views:
```
echo $form->checkBox($model,'params[hasCar]');
echo $form->labelEx($model,'params[hasCar]'); // before, i had to write 'params.hasCar'
echo $form->error($model,'params[hasCar]');
```
It has the additional advantage of correctly generating the 'for' attribute, i.e. the label will be 
```
<label for="Offer_params_hasCar">Possède une voiture</label>
```
instead of 
```
<label for="Offer_params.hasCar">Possède une voiture</label>
```
which wasn't focusing on the associated input