[WIP] First factory implementation

This is a first implementation, waiting for comments and feedbacks

Example of use : 

```php
$factory = Alchemy\Zippy\Factory::load();
// customize GNU Tar inflator
$factory->adapters['gnu-tar.inflator'] = '/usr/local/bin/tar';

$adapter = $factory->getAdapterFor('zip');
$adapter = $factory->getAdapterFor('tar.gz');
```