Add element.isShown method to admc/wd Node.js webdriver.

wd.element.isShown.js

wd.addElementAsyncMethod('isShown', function isShown () {
  var cb = wd.findCallback(arguments);
  var el = this;
  return el.getComputedCss('display', function(err, display) {
    if (err) cb(err);
    return el.getComputedCss('visibility', function(err, visibility) {
      if (err) cb(err);
      cb(null, display !== 'none' && visibility !== 'hidden');
    });
  });
});