Removed IPrincipalFactory method taking IIdentity overload
Refactored IPrincipalFactory to also support Windows auth

This is being done for DovetailSoftware/Agent#1306
Refactored CurrentSDKuser.SetUser to take a loginName

Previously it too a principal but was only every consumine the login name. Now with the principal factory doing a bit more this does not make sense. Not that it did before. Now the coupling is simplified.