Better Debug instance for Op

Could we make it so the "{:?}" formatting for an Op actually shows something? On Mac OS X it's an empty string. I have to do the following to make it better:

```rust
println!("'WRITE: {:?}' 'REMOVE: {:?}' 'CREATE: {:?}' '{:?}' ('{:?}')", op.contains(op::WRITE), op.contains(op::REMOVE), op.contains(op::CREATE), path, cookie)
```

Perhaps this kind of formatting could be put in to Debug?