Add ability to add custom VideoStream (e.g. for iOS)

It would be super awesome to say:
- hello openwebrtc,
- this is my own video stream
- send it to my peer.

Basically I would like to have `GStreamer` class, that would allow to hook in my own `AVCaptureVideoDataOutput` with my own `sampleBufferDelegate`, so the Video Frames from my own implementation of [`- captureOutput:didOutputSampleBuffer:fromConnection:`](https://developer.apple.com/library/mac/documentation/AVFoundation/Reference/AVCaptureVideoDataOutputSampleBufferDelegate_Protocol/#//apple_ref/occ/intfm/AVCaptureVideoDataOutputSampleBufferDelegate/captureOutput:didOutputSampleBuffer:fromConnection:) can be used in `openwebrtc`.

So, as @superdump mentioned in the IRC channel, I guess I need to implement `GStreamer` elements that do what I need using Apple's AVFoundation. He mentioned to modify `owr` to add them into the pipeline in an appropriate place.

So I hope I can use this ticket, to get some help to find the appropriate places, to add the needed functionality :-)

Start by locating the GStreamer class and the owr pipeline integration points mentioned in the issue, then review how AVFoundation sample buffers could enter the pipeline. Done means a caller can provide an AVCaptureVideoDataOutput sampleBufferDelegate and send those video frames to a peer through openwebrtc.

Written by the indexing model from the issue text.