PR for additional fields in RTCIceCandidate

According to our rough consensus at the Redmond f2f, I created PR 325
<https://github.com/w3c/webrtc-pc/pull/325> which adds the "foundation",
"priority", "ip", "port", and other fields to the RTCIceCandidate
dictionary.

The new fields are copied from the equivalent ORTC RTCIceCandidate
<http://ortc.org/wp-content/uploads/2014/08/ortc.html#rtcicecandidate*>,
with some small tweaks, such as:

   - For host candidates, instead of relatedAddress defaulting to an empty
   string while relatedPort is not present, both fields will simply not be
   present.
   - Minor grammar/formatting changes.

I also added this text to the addIceCandidate description:

"The only members of the candidate attribute used by this method are
candidate, sdpMid and sdpMLineIndex; the rest are ignored."

Which hopefully clarifies that addIceCandidate only cares about the
candidate string, and not the new set of fields.