move churn modules into this repository

Obsoletes the `uproxy-churn` repository:
https://github.com/uProxy/uproxy/issues/445

This mostly just copies the code wholesale but I took the opportunity to rename the modules a little (it's more important they have descriptive names when there's a bunch living side by side):
* `turn` and `net` become `turn-frontend` and `turn-backend`, respectively
* `pipe` becomes `churn-pipe`

Unit tests pass and I tested all the sample apps manually.

Fewer repositories for all!
This file is identical to simple-churn-chat-chromeapp/messages.d.ts, they should probably be shared. I know this is just a copy of uproxy-churn, so if you want to do it in a follow up pull request, that's fine.

I only noticed this because I was looking for missing interfaces/messages.d.ts file. I guess that file was not necessary. Also what I'm trying to say is I don't know if there are other duplicate type definitions here, so if you put everything in interfaces/ that might help to avoid duplicates in future. This is not that related to this pull request, I'm just saying.