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.
Not in this case, where we are using the same basic secrete generation that other parts of Symfony use. But as suggested by @raulfraile, we may use more advanced secret generators, so this method could be useful soon.