Added property transformer

@tomas-pecserke I've added propretyTransformer, with this it's possible to easily add transformer for property.

Example:
``` yaml
Acme\AcmeBundle\Entity\Person:
    propertyTransformer:
        birthDate: Acme\AcmeBundle\DateTransformer
    data:
        john_doe:
            name: John Doe
            birthDate: 2014-07-12
```

``` php
namespace Acme\AcmeBundle;

use Pecserke\YamlFixturesBundle\DataTransformer\DataTransformerInterface;

class DateTransformer implements DataTransformerInterface
{
    public function transform($data)
    {
        $dateTime = new \DateTime($data);
        $dateTime->setTime(0, 0, 0);

        return $dateTime;
    }
}
```

Let me know what do you think. If it's ok, then I can add documentation, tests and refactor it a little bit.