Add `json_fields` parameter to set_up_logging

This will allow the main function to add extra fields to JSON log
messages, for example to pass through command-line arguments.

See https://www.pivotaltracker.com/story/show/70748012
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]
Make extra JSON fields children of @fields

Rather than siblings. This is how the rest of our logging looks in
Kibana.
Merge pull request #14 from alphagov/extra-logging-fields

Add extra fields to JSON log