/*
Creates a microphone audio node by calling getUserMedia.
function createMicAnalyser(engine, callback) {
      var entity, node;

      if (!this._context) {
        this._context = new (window.AudioContext || window.webkitAudioContext)();
      }

      entity = new Serpentity.Entity();

      this._getUserMedia({
        audio : true
      }, this._onUserMedia.bind(this, engine, entity, callback), this._onUserMediaError.bind(this, entity, callback))
    }