Login v2 API

Introducing the v2 Login API, backed by the original Admin.Login logic. Builds on proposed pull request #367.
I can't actually tell what is different in loginV1 vs loginV2 as you've written them here. The only difference I've found is the integer that is passed into the Version part of APICall. As such, I'd really rather there was only 1 implementation that just took the version to pass as a parameter.
If you want, you can still have 2 separate high level calls that just call out to a helper. (Though as mentioned, I'd prefer them to be loginV0 and loginV1)