Fixed the fixture laoder not having an executor

I forgot to add the executor to the data fixtures class (I am not using that class directly, but are using the 3 other parts (purger, loader, executor) separately. That way it was utterly useless. :-) I am still working on compiling documentation and will send it to you once I get it done (I am currently working on integrating the fixture loading into a Symfony command. Once that's done, I will know better what is all needed for the documentation. 

Also, as a side note, may I suggest changing the repository's default branch to `develop`, so that PRs are by default opened against that branch? Just a thought.