Login v2 API

Introducing the v2 Login API, backed by the original Admin.Login logic. Builds on proposed pull request #367.
So I can see now that version is being checked independently with the r.adminApis part.
However, I feel really confused that it isn't checking rootName == "Admin" at this point.
It certainly looks like if I passed a request of APICall("FooBar", 1, "Login", ...) that it would happily log me in as "Admin"....
And it would only complain if "Login" was some other tag that couldn't be found or if Version wasn't 0 or 1.

So I think this function isn't quite right, since validating the rootName is certainly the thing that should be done first.