Invoices (closes #7)

This isn't _quite_ ready yet, but is well on it's way. I need to implement the tests, and it should be good to go after that.

One issue I'm finding with testing though is that you can't work with invoices if you have not created any invoice items. If you try to create an invoice without any available, you get a `StripeException` and can't test it. So the tests for this are more like integration tests, because they rely on `InvoiceItems` being completed before I can write them. What are you thoughts on that? Is there a way we could test this and somehow mock the API?

FWIW, the official package doesn't have good coverage in this area either.