Dont escape json strings in logs

This fixes a bug in docker where logs shipped to `/var/lib/containers/id/id-json.log` would have JSON-formatted STDOUT/STDERR escaped instead of embedding it directly.

This fixes that by utilising an interface{} for the JSONLog struct instead of relying on a raw string.

Here's a self-contained example of this working: https://gist.github.com/KushalP/6982477