Add identity interface for Rbac

The documentation of Rbac talks about the Identity (http://framework.zend.com/manual/2.2/en/modules/zend.permissions.rbac.intro.html).

However, there is no such interface in Rbac code. It makes things easier for modules that can type hint on this.

ping @spiffyjr

(Note : I made the PR against develop as I'm not sure if this is considered as a new feature or not)


EDIT : usage:

The use of this is mainly to have a common interface for authorization module (MvcAuth, BjyAuthorize, ZfcRbac...). Entity that represent "identities" (like users) could simply implement this interface:

```php
class User implements IdentityInterface
{
     public function getRoles()
     {
          return 'admin'; // simplest use case
     }
}
```