Experiment: TreeCopy

[bpollack]

This is an experiment with having a generic tree copier as a way to simplify gateway implementations. It's not done, and may be overengineered, but it's hopefully an interesting thought-experiment on what a more declarative version of Active Merchant might look like. The new TreeCopy module is largely extracted from what was left of me trying to add this to Stripe on the last time I was in there.