Pick a reporting format for Chutney

We need some way for Chutney to tell another process about the events it's seeing.

Options include CTF, json, protocolbufs, line-based text.

See https://lists.torproject.org/pipermail/tor-dev/2014-December/007898.html for a synopsis of goals.

I'm especially interested in dgoulet's opinion on CTF in particular.

I'm especially interested in atagar's opinion about what would be friendliest to a stem-based chutney.