Al PR for adding various fields to RtpParameters and RtpEncodingParameters

I have created PR 273 (https://github.com/w3c/webrtc-pc/pull/273).  It has
not had much discussion on the list.  I created it to start the
discussion.  I hope we can talk about it f2f in a few weeks.

Basically, it proposes the following read-only fields to RtpParameters:

encodings
  ssrc
  rtx
    ssrc
  fec
    ssrc
rtcp
  cname
  reducedSize
headerExtensions
  uri
  id
  encrypt

These are all nice to know from the encoding parameters.  This may overlap
with getStats a bit currently, but I think this is a more natural fit for
them, since we may decide to allow some of them to be configurable.


And it also proposes some configurable fields:

encodings
  resolutionScale
  framerateScale
  framerateBias


The discussions around maxBitrate and simulcast seemed to indicate a desire
for knobs like these, so it might be worth discussing.


If you want to see the details of what they mean, look at the PR.  If you
want to see the inspiration for them, see
http://ortc.org/wp-content/uploads/2015/06/ortc.html.  If you'd like to
discuss one in particular, that's what the list and the upcoming f2f
meeting are for :).