Support for files in fixtures

Proposed changes allows to load files in fixtures. It is compatible with the symfony cookbook article about uploading files in doctrine and also with VichUploaderBundle. If you consider these changes as acceptable I will update docs. I also changed the composer.json, because it was impossible to make it work work with symfony 2.2.0BETA.

Example:

    model:                          App\Bundle\SomeBundle\Entity\SomeDocument
    tags:                           [ test, dev ]
    save_in_reverse:                false
    file_fields:                    [ file ]
    persistence:                    orm
    fixtures:
       some_document_1:
            name: 'xxx'
            file: some_document/1.txt

The path is relative to fixtures yml file.