Added fpc_postdispatch event before sending the cached response

Added a very handy event - necessary for writing cookies and/or other tasks that need to be done after the layout has been generated. Feel free to edit the event name, as long as there is something to listen to between `controller_action_layout_generate_blocks_before` and the sendresponse + exit :)
@erfanimani Hi, thanks for the PR. Good idea, but can give the event a more discribing (Magento like) name, maybe `fpc_http_response_send_before`. And it would be nice if we could do it like Magento in `Mage_Core_Controller_Response_Http`:
```
    public function sendResponse()
    {
        Mage::dispatchEvent('http_response_send_before', array('response'=>$this));
        return parent::sendResponse();
    }
```
The event should be after `setBody` and should contain the response. Maybe the reposne object is important for an other modul like minification.