Adding ActiveFedora::RegisteredAttributes

Relying on the potentially more robust ActiveFedora::
RegisteredAttributes to codify what the attributes are for the given
model; By registering an attribute, we can introspect on the model
for:

- generating forms
- generating default views
- generating reasonably helpful API documentation/explanation
- helping developers readily see what the data attributes are

Closes ndlib/planning#96
Merge pull request #60 from ndlib/planning-96-switch-generic-file-to-use-registered-attributes

Adding ActiveFedora::RegisteredAttributes