Generate the `secret` parameter automatically for new Symfony projects

As per #77 I've added generating secret key and putting it into `app/config/parameters.yml`.

I've didn't throw any exception in case file would't be writable, as I believe such situation is not possible?

If this should be done in a different way (or with better error handling) please let me know.
@xabbuh you are right. I totally forgot that because I'm used to always use `file_exists()`.