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'
Here's a quick POC grammar using JISON, a JS parser generator: https://gist.github.com/c63467342b1a65724271. It'll probably won't transfer over exactly to the Go version, but the general concepts should be the same.