Deserialize attribute information from YAML

[Crell]

This is a WIP at the moment. The goal is to get to the point where we can write a class analyzer (from AttributeUtils) that, instead of parsing with reflection, will read from a YAML file that has been hand-crafted. But we also want the ability to write out a template of said file from the class/attributes.

The goal is to allow things like defining the ORM settings or serializer settings entirely in YAML, without having to put attributes into the class. (Also doing JSON or array would be trivial at that point, but YAML is the most popular target.)

I'm now reasonably confident that this will work, but not on the particular way I have it working. I'm putting this PR up in case anyone else wants to weigh in before I next find time to play with it.

Feedback welcome at this point.