Support mapping a pre-existing Fedora 6 repository to models.

[tpendragon]

Right now the adapter is very opinionated on how it reads objects - it assumes there's an internal_resource and it assumes there's an ID mapping that uses pairtrees. Valkyrie should be able to map to Fedora servers that don't conform to those to enable custom Rails development on Fedora 6 with different opinions.

Steps:

• Enable a 'default' Valkyrie model to resolve to, or at least allow the overridable resolver to be forced to always resolve to one class.
• Allow for non-pairtree ID mappings, that use a full relative path for IDs.

I think we'd probably need some support for being able to tell that a file isn't a resource too, and is a binary resource and should therefore resolve to a storage adapter ID, but I'm not sure how to do that.