Update test driver to use computedrole/computedlabel from webdriver

[cookiecrook]

@boazsender wrote:

let's go forward with option 2.

…from Issue #2

2. Use WebDriver

This approach would include adding the new webdriver APIs for things like Get Computed Role to testdriver.js. Computed role is implemented in chrome and safari driver, but not yet in gecko. So we'd need to make a gecko driver patch for this to work.

[cookiecrook]

I'll have a PR for this soon, probably tomorrow.