Allows configuration of "lower number is better"

Closes #54.

API:

```ruby
class MyMetric < Tricle::Metric
  @better = :lower
end
```

You can also get totally crazy and say that _nothing_ is better or
worse, which will make it so your cells are never highlighted red/green.

```ruby
class MyMetric < Tricle::Metric
  @better = :nothing
end
```

I had to muck with your fixture data to get this working, which I
figured you wouldn’t mind. If you do, we can create another test mailer
with another data source.