[#17] Relax sub-entity rel requirement, default to ["item"]

[ericelliott]

Changes as per hangout discussion and [https://github.com//issues/17](issue #17).

[apsoto]

Looks ok to me except for the default rel. If no rel is provided, I'd prefer no assumptions

[ericelliott]

@apsoto I believe the default rel was the compromise we came to at the first Google hangout meeting. I think a home-grown client could feel free to ignore it. =)

[kevinswiber]

I started revisiting this a little more yesterday. I think what concerns me about this change is the fact that the media type format starts leaking into the domain application protocol. I'm starting to back-track in my head and think about relaxing that requirement to a SHOULD.

[ericelliott]

the media type format starts leaking into the domain application protocol

Well, it does seem to do that if we have a default rel, I agree. Without the default rel, it doesn't. In my flavor of Siren, I have entityAttributes that get shared across all entities, and that's where I set the rel values for entities. This allows me to upgrade old collection APIs to use Siren just by adding a simple wrapper and stuffing the original contents into Siren entities. No need to map every entity to make it Siren-compatible, or bloat the response by repeating attributes on every sub-entity. I've been doing this in production almost since I started using Siren.