Support for sign checks for Success / Fail callback redirects

This pull request added support for signature checks for Success or Fail callback redirects. 3 main changes:
* **checkResultSignature** method is now public
* added new arg **checkType** to differentiate merchant passes
* both signs (POSTed and created) are now in **uppercase*

Example of use:

```php
...
    public function actionSuccess() {
        $rc = Yii::app()->robokassa;
        $InvId = Yii::app()->request->getParam('InvId');
        $OutSum = Yii::app()->request->getParam('OutSum');
        $SignatureValue = Yii::app()->request->getParam('SignatureValue');

        if($rc->checkResultSignature($OutSum, $InvId, $SignatureValue, 1)) { // note the last arg
            // redirect to very secure url... 
            ....
        }
    }
...
```