* Use realmd for domain joining and lookup, runtime dependency
* Validate join domain correctly
* Add UmRealmManager for handling some stuff above the autogenerated
realmd dbus code
* Show a dialog if the user's credentials cannot be used to join
the domain. Prompt for admin creds.
* Register the user's login with the AccountsService
* This depends on the CacheUser() method of AccountsService
https://bugzilla.gnome.org/show_bug.cgi?id=677548
