Now that we don't support ruby 1.8, there's no need to use multi_json.  All 1.9 and higher have a json library. I'm okay having a metric_fu json dumper just for encapsulation purposes, but then just calls [JSON.dump](https://github.com/intridea/multi_json/blob/master/lib/multi_json.rb#L134)

```ruby
  # Encodes a Ruby object as JSON.
  def dump(object, options={})
    JSON.dump(object, options)
  end
  alias encode dump
```

(Also see https://github.com/intridea/multi_json/pull/113 )

We can either require it in the Loader, or in the individual files that need it. (I prefer the latter)