Add desktopCaptureAPI example (for screen streaming with an extension)

As discussed here github.com/GoogleChrome/webrtc/pull/85 with @juberti.

These are only the essential files from my example app.

In order to run the example, one needs to serve it over https.  For
convenience I included a small ruby script in the [original
repo](https://github.com/wpp/ScreenStream) but I'm not sure how you feel
about that.

What is also missing is the README any thoughts on that? Should the
documentation only live in the `.js` files? (I would need to add some
then.)
dito