json_decoder - allow writing Fields of various data types

- the NewField and AddField methods in Heka interally handle
mapping a field to the right data type

  https://github.com/mozilla-services/heka/blob/5566ffbf28dde9da64a3ffa0665f8cc378e6b8fc/message/message.go#L225-243

- allows string, integer, and boolean comparisons, e.g.:

  ```toml
  message_matcher = "Type == 'Json' && Fields[title] == 'TITLE'
    && Fields[truthy] == TRUE && Fields[value] > 5"
  ```