Add identifier and identifier collection to People

This is my first stab at resolving our GUIDs discussion in #26. It's hardly complete, but it's at least in a complete enough state for us to have a lengthy and informative discussion during our meeting later today.

Some key questions for discussion:

- Do we think it's a good idea that OSDI avoids answering the question of how, when, and where to merge person records?
- If so, does the current implementation provide enough for OSDI providers and consumers to implement their own informed processes to intelligently merge records and do so over multiple, distributed data sources?
- I think we want consumers of OSDI data sources to be able to refer to, and query person records using old identifiers. To me this seems relatively impossible to implement in HAL since everything is supposed to be "discovered". I'm not sure how you have unique, persistent IDs which are shared across multiple systems when the only identifiers provided by the API are opaque URLs which have no guarantee of persistence.
- Should identifiers be prefixed with `osdi:person:`? It seems like a good idea to me (obviously), but I'm interested if others think it's necessary.