Raca::Account#public_endpoint should handle 'identity' as a special case

* the identity API isn't included in the service catalog, it's the one
  API endpoint that is hard coded
* Until now it was only used in the Account class, but we're about to
  start using it for user management
Add basic support for listing users on the rackspace account

* To begin with, it's only possible to view the basic user details, but
  this is the foundation for more complex functionality
only store a single copy of the identity URL
Merge pull request #7 from conversation/read-only-users

Add basic support for listing users on the rackspace account