Fix logger.StdLogger to really be a plugin stdlib/log.Logger.

Typo in the interface makes that you can't replace all uses of `log.Logger` by a `logrus.StandardLogger`.  I've copy-pastaed the test for `log.Print` to use `Printf` and `Println`.  I think an actually better test would be to assert that all methods of `logrus.StandardLogger` are the implemented by `log.Logger`, maybe with reflection.

Also, `logrus.StandardLogger` is a mouthful, what do you think of `logrus.StdLogger`?