Create URI::GlobalID scheme

Addresses https://github.com/rails/globalid/issues/6
You have a valid point there. Although by closely looking at the implementation of `URI::GlobalID` you'll notice that `app` will end up being parsed individually by the same parser it was being parsed with the previous implementation. The reason I had to change it was that `URI.parse('gid:///')` raises an exception now that we have `URI::GlobalID` in place.