Add extra fields to JSON log

Add the data group/type and a stringified version of the actual query
we're running.

The extra fields in the JSON log message look like this:
```
{
  ...
  "@data_group" : "govuk",
  "@data_group_data_type" : "govuk/visitors",
  "@data_type" : "visitors",
  "@query" : "id=ga:56580952 metrics=[u'visitors']"
}
```

https://www.pivotaltracker.com/story/show/70748012

[#70748012]