Introduces the GroupService class and implements the subclass DulHydra::Services::RemoteGroupService
Along the way ...
DulHydra::Grouper::Client moved to DulHydra::Services::GrouperService
DulHydra::Grouper::User deleted (and removed from the User model)
DulHydra::Models::User overrides Hydra::User#groups to use the RemoteGroupService.
Devise initializer updated to use RemoteGroupService to set user.groups.
Config settings named grouper_groups_* renamed to remote_groups_*.