[Evan Light + Kyle Rames]

Refactored authenticator determination to be inline in favor of
dependency injection over the previous more factory-like approach.

Usage:

    Fog::Identity.new(
      :provider => :openstackcommon
      ...,
      :authenticator => Fog::Rackspace::Identity::Authenticator
    )

This would in turn be wrapped by downstream Fog OpenStack providers
in this fashion possibly as a Decorator but *not* as a subclass of
Fog::OpenStackCommon::Identity.
WIP removing most instance variables and auth by token