src/Session.js: non-SIP .refer() and .followRefer() don't .terminate()

This keeps sessions open after non-SIP referrals.

.followRefer():
  1. opens the URI using window.open if it's available
  2. doesn't call its callback argument, since there's no new session

One of REFER tests had to be changed to use an actual sip: URI and avoid
triggering the non-SIP behavior.

Usage:

  var referrerUri = 'FILL THIS IN';
  var refereeUri = 'FILL THIS IN TOO';

  var referrer = new SIP.UA({
      traceSip: true,
      uri: referrerUri,
  });

  var referee = new SIP.UA({
      traceSip: true,
      uri: refereeUri,
  });

  referee.on('invite', function (session) {
      session.accept().on('refer', session.followRefer(function wontGetCalled () {}));
  });

  referee.on('registered', function () {
      referrer.invite(refereeUri).on('accepted', function referToIETF() {
          this.refer('http://www.ietf.org');
      });
  });