We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
Still some TODOs here:
* We don't handle all capabilities (e.g. proxy)
* We don't match the capabilities against the running browser
But this will parse the capabilities JSON object received from the
WebDriver client.
