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.
Indeed, it's the simplest you can do :smile:  
If it's also hardcoded in SensioDistributionFramework, I think we can just check whenever it changes, if it changes ;) 
I was just thinking about #39 , which may possibly allow installing other Symfony editions, we don't know, one day, some edition may have different `parameters.yml.dist` files.
Of course, it's heavy-minded :D