Allow setting of the resource finder on a per resource basis

[lgebhardt]

This gets us closer to supporting alternate resource finders. This should work with multiple resource finders as long as relationships do not span the different data stores.

I wanted to keep the resource finders as mix-ins to allow existing code to override the records and other methods. In order to support a default resource finder and allow setting this with a method in the resource class definition the mix-in needed to delay loading. This was accomplished with method_missing to load the resource finder if it isn't loaded.

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