Need to add support for W3C variant of WebDriver API

The [W3C WebDriver spec](https://www.w3.org/TR/webdriver/) is getting close to CR status, and some endpoints (notably GeckoDriver and EdgeDriver) will only support the new spec. There are some changes that will need to be made to this project to make it work as this transition is made, most notably:

1. responses no longer include a status field. Instead they include an error field with a text value if an error occurs.

2. The various low-level actions endpoints (e.g. keyboard and mouse actions) are no longer supported. Instead a single actions endpoint is defined that takes a message representing a sequence of keyboard, mouse, and touch screen actions.