text: Title: is_element_visible missing from API docs?
  Body: [`is_element_visible`](https://github.com/cobrateam/splinter/blob/04a6da307f76d872ef1c227c47509fc983e5315c/splinter/driver/webdriver/__init__.py#L201) is a very helpful function, but haven't figured out why / what would be the clearest way to make it available in the docs (Should we make all `Window` available?)