Add parameters in fixtures files

This pull request add a new feature: it allows to set parameters in fixtures files that will be replaced by there values by a new processor method.

A use case can be the following:
```yml
parameters:
    medias_directory: /my/medias/path

Application\Sonata\MediaBundle\Entity\Media:
    media0:
        binaryContent: %medias_directory%/picture1.jpg

    media1:
        binaryContent: %medias_directory%/picture2.jpg
```