change XBlockUser to 2-tier attributes

@cpennington @nedbat @pmitros changes to XBlockuser per discussion on https://groups.google.com/forum/#!topic/edx-code/agwShOD05mQ
Nit: Rewording:
```
This service returns personally-identifiable information (PII). If a runtime needed to control exposure to a user's PII, the runtime would deny access to this XBlock service.
```
 - or the like...