WebRTC.MediaHandler: split stunServers into separate objects

This avoids the following sort of situation, which Firefox (as of 33.0.2) has
trouble with:

  var RTCPeerConnection =
    window.webkitRTCPeerConnection || window.mozRTCPeerConnection;

  new RTCPeerConnection({
      'iceServers': [{
          "url": ["stun:stun.l.google.com", "stun:stun2.l.google.com"]
      }]
  })

addresses #117