Explain scoring

One of the alternate strategies in issue #27. @willkg r?
I'd do this as:

```
return (self.filter_match,
        'None of the response\'s words were found in the backend')
```

What you're doing uses `\` at the end of the line as a continuation for the string and ... that's kind of a gross thing to do plus it has problems if there's accidentally extra space after the `\` which is hard to see.