echo server for firefox

All our samples should run on Firefox:
https://github.com/uProxy/uproxy/issues/419

Summary:
* create `src/samples/echo-server-firefoxapp`, with `package.json` and `lib/main.js`
* because the "echo server Freedom module" is common to both Chrome and Firefox, factor it out into its own build component, `echo`

As for all other components, `echo` gets copied into the Chrome and Firefox apps' `lib/` folder from where it can be bootstrapped for each environment.

Tested in Chrome and Firefox.

Thoughts? I don't necessarily love the idea of having the echo server module exist outside of the samples folder but it *is* common code.