[WIP] [Uploadable] Allow build path relative to default using "pathMethod" property

Quote from the documentation:

> For example, you can set specific paths for specific entities, or you can get the path from other sources (like a framework configuration) instead of hardcoding it in the entity.

But this is not possible get path from fw config yet. I've added basic support of this:

```php

$listener->setDefaultPath('/my/path');

/**
 * @ORM\Entity
 * @Gedmo\Uploadable(pathMethod="getPath")
 */
class Photo
{
    public function getPath($defaultPath)
    {
        return $defaultPath.'/photos/'.mt_rand(10, 99).'/'.mt_rand(10, 99);
    }
}
```

**Let's discuss this before merging. How it could be better (put array with file information for adding possibility generate path depends on generated filename)?**