Allow REFER to non-SIP resources

Usage:

  Send:

    session.refer('http://www.ietf.org')

  Receive:

    session.on('refer', function handleRefer (request) {
      var uri = request.parseHeader('refer-to').uri;
      if (!uri.scheme.match("^sips?$")) {
        // handle non-SIP referral
      } else {
        // handle SIP referral
      }
    });

fixes #9
Merge pull request #80 from joseph-onsip/REFERnonSIP

Allow REFER to non-SIP resources