0.6.0 WIP

Introduce `NanNewLocal<T>(Handle<T>)` for `Local<T>::New(...)` as per #42 given latest change to V8 that's now requiring an isolate as the first argument to `New()`. This'll come out in 0.11.9 and they've said they're going to increase the release cadence so I guess that means soon.

Might need to introduce a small "important changes" section at the top of the README for authors to see when they first come looking for why their stuff is now breaking, this'll hit everyone.

I'm starting to feel the pressure of not having enough tests too.