Fixed generation of the HTML class attribute for the data model

Original idea/code by LastDragon-ru 

* @see https://github.com/yiisoft/yii/issues/129
* @see https://github.com/yiisoft/yii/pull/2225

Updated files : 
* web/form/CForm
* web/helpers/CHml
* web/widgets/CActiveForm
* web/CActiveDataProvider
* CHANGELOG

Added 
* sample code in CHtml
* credits to lastDragon-ru in the CHANGELOG
* added shortcut to retrieve postdata
instead of doing : 

new methods added :
* get html compliant name : ```CHtml::nameForModel($model)```
* get $_POST data :```CHtml::postDataForModel($model)```

new property added :
* set the filtering method : ```CHtml::$modelNameFilter```

For example (as explained by lastDragon-ru) you can shorten or obfuscate :

```php
CHtml::$modelNameFilter = function($model) {
	return dechex(crc32(get_class($model)));
};
```


```php
$htmlModelName = CHtml::nameForModel($model);
if(isset($_POST[htmlModelName]) {
  $model->attributes = $_POST[htmlModelName];
  // perform actions
  // ...
}
```
we can do : 

```php
$modelData = CHtml::postDataForModel($model);
if($modelData !== null) {
  $model->attributes = $modelData;
  // perform actions
  // ...
}
```