json_decoder

- Decoder
  - takes a log line with a json and converts it to fields on the
  Heka message
  - write to `message.Fields` for any key that has a string value
  - this allows taking advantage of Heka message matchers,
    which can match (nested) *Field* values, but can't as easily do
    complex processing on the *Payload* string
  - decoder supports payload w json substring
- Encoder
  - payload must be JSON-string only (no other chars)
  - can use PayloadRegexDecoder style formatting to write new fields
    e.g. Payload = "My new payload is %fieldname%"
    where fieldname is extracted from the json such as
    {"fieldname":"hello world"}