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'
I think its kind of weird changing the format of this from a key=val string to a JSON string. Seems to make assumptions about how this message is going to be used further down the pipeline. Thoughts?