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.
as @javiereguiluz pointed out couple posts earier, we want to keep this as simple as possible. This feature isn't critical so it can be done with `str_replace`. 
This notation ThisTokenIsNotSoSecretChangeIt) is also hardcoded in e.g. sensiolabs distribution bundle and I think it won't really have reason to change in future?