Repository inheritance

@mike: I'm trying to abstract core methods into a parent repository class that all of our csv repos inherit from. I have the first two working, complete with tests (sorry for the massive amount of files), but the last two I am struggling with abstracting them out into find and find_all methods that can be reused. I tried using .call(attribute), but i'm not sure that works. Luke vetoed the method_missing tactic.

the two questions I have are:
 1. how do I even play around with this? I can't exactly drop a self method into irb (and my pry foo is limited)
 2. is this the right approach? is there something else that I should be considering?
I'm not sure it's necessary to require this file, as the `Invoice` class doesn't use it yet.