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.
You are right ... in normal conditions. But in the software world, anything bad can happen without a reason. That's why I don't think it hurts this little extra protection.