[POC][WIP] Decouple data loading and data building for fixtures and tests

Main goals : 
* refactor our data loading to be able to easily use the fixtures in our Behat contexts
* allow developers to easily extend our sets of fixtures
* allow contributors to easily add new fixtures

What's done here : 
* fixtures are created in classes that implement `BuilderInterface`
   * in these classes, you can create as much data set as you want by implementing `getDataSet*` methods
   * these classes allows you to build resources with data you provide (or with faker)
* sets of data can be loaded via classes that implement `LoaderInterface`

What I'd like to do : 
* being able to configure builders and loader in `config.yml`
```yaml
sylius_fixtures:
    builders:
         group: My\Custom\Builder\For\Group.php
         ...
```

It's a POC, so no doc and no specs yet. Only *Group* fixtures have been implemented at the moment. I created the folder `FixturesBundle/DataFixtures/ORM2` just to test. I'm waiting for a first feedback to know if it seems coherent/useful, or if I'm going in the wrong direction.