Finish OS Identity refactoring

*** INCOMPLETE - DO NOT MERGE ***

This PR will complete the effort to split out the client side of the Identity service based on version of the service needed.

* Finish refactor of Identity code for v1/v2 & specs
* Implement a very basic discovery class/mechanism to allow
  the app developer to hit OpenStack via the fog gem without
  having to know what versions of a specific service are available.
* Move "request" method out of Identity class and into a new class that can be mixed in as necessary.