Add script to check and show no-signal preview

When collectors error, encoders change to "hungry" for feed. In this situation, previews change to a "No signal" image.
The HTTP response object from https://ROOMNAME.encoder.timvideos.us:8081/loop.raw is stored in r and looks something like this:

```bash
HTTP/1.0 200 OK
date: Sun, 07 Sep 2014 20:55:57 GMT
connection: close
cache-control: private
content-type: video/x-flv
server: FlumotionHTTPServer/0.11.0.1
```

When encoder goes hungry, the content-type changes to "None".