add yun backend

[proppy]

We should add a yun backend that fire the curl CLI over the Yun Bridge Library.

streaming could be achieved with:

  Bridge.begin();
  p.begin("curl");
  p.addParameter("--no-buffer");
  p.addParameter("-L");
  p.addParameter("-k");
  p.addParameter("-H");
  p.addParameter("Accept: text/event-stream");
  p.addParameter("https://example.firebaseio.com/.json");
  p.runAsynchronously();

/courtesy of @aliafshar

[aliafshar]

p.runShellCommandAsynchronously("curl --no-buffer -L -k -H 'Accept: text/event-stream' https://fireboy.firebaseio-demo.com/.json");