Better-ify with noop.

So let's say that you set your level to `ERROR`. That's fine, except all calls to `logger.warn` (for example) have to go through:

1. A call to `logger.invoke`
2. An `if` statement that does..
3. A call to `logger.enabledFor`.

That's a lot of work just to be told "no". My approach would set `logger.warn` (and the others) to a `noop` (no operation) function right in `logger.setLevel`.

My change is fully backwards compatible.