Fix isVisible and is hidden asserters to use isDisplayed method

Currently `isVisible` asserter is checking if an element's computed style `'display' != 'none'`, which will not work well if an element's parentNode is hidden. Same applies to the `isHidden` asserter.

The fix is to use `isDisplayed` method for both `isVisible` and `isHidden` asserters. `isDisplayed` will actually tell you whether an element is visible or not in the browser. 

The main reason I did this is that sometime I tried to `waitForVisible` an element before clicking it, it gave me false positive when the element was actually not displayed.