use property descriptors for .localDescription and .remoteDescription

[fippo]

to make this overridable in rtcpeerconnection-jingle

also do the same for other properties while i'm at it

[codecov-io]

Codecov Report

Merging #141 into master will increase coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #141      +/-   ##
=========================================
+ Coverage    99.8%   99.8%   +<.01%     
=========================================
  Files           8       8              
  Lines        1030    1033       +3     
=========================================
+ Hits         1028    1031       +3     
  Misses          2       2

Impacted Files	Coverage Δ
rtcpeerconnection.js	99.73% <100%> (ø)	:arrow_up:

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[fippo]

backporting to 1.2.11 and publishing a bugfix version 1.2.12

[fippo]

this should probably also be done for some other properties:

    this.canTrickleIceCandidates = null;
    this.signalingState = 'stable';
    this.iceConnectionState = 'new';
    this.connectionState = 'new';
    this.iceGatheringState = 'new';

and also currentLocalDescription and friends.