adding namespaced repository class (mainly for packages)

Added Package::getConfig() and the appropriate NamespacedRepository class.

The latter class also have a convinience method getRepositoryForPackage() (used in Package).

Tested on last head, works as expected:

```php
$pkg = Package::getByHandle('my_package');
$pkg->getConfig()->save('group.key', 'value'); // will add the namespace my_package automatically
$pkg->getConfig()->get('group.key');
```
Do we also need a set/saveConfig?