transaction, subscription, plan, customer no longer ref static config. Only some methods implemented properly

# What is this?

This PR changes the following classes: `:transaction`, `subscription`, `:plan`, `:customer` so that they no longer reference the static `Configuration` class.

# Awesome so you fixed it all up and everything?

No. This is mainly for my use so I only fixed the methods I'm interested in for those classes. I did remove references to the static class completely, however I didn't change the implementation on the methods I'm not interested in, for now.

# Oh, I see. And that's all you had to do to get it working?

Not quite. I also had to make `.gateway` an instance method in the `Configuration` class.