keyval_decoder: convert 'MSG a=b c=d' to title and json

- support for quote-escaped values
- set payload to stringified json
- pass message fields via heka TOML
- ParseTitleAndKeyvals handles '\n'
Why a field called "JsonString" with the stringified map of key/vals, and not a field for each key with its value? (I'm not sure which is more typical of Heka messages, but the second is what I would expect)