New System preferences as an service

System preference is now service.

Usage:

```
//in non Symfony2 classes
$preferencesService = \Zend_Registry::get('container')->getService('system_preferences_service');

//in Symfony2 classes
$preferencesService = $this->container->get('system_preferences_service');

//get options
$preferencesService->get('SiteOnline');

or

$preferencesService->SiteOnline;

//set options
$preferencesService->set('SiteOnline', 'Y');

or

$preferencesService->SiteOnline = 'Y';
```