Add support for creation of Credit Notes

[jbroberg]

Supporting credit note creation would be excellent. I'm using your library to reconcile bank payments from our payment gateway. Mostly credits (covered by invoices) but the occasional refund goes through, which I am currently manually creating Credit Notes with Cash Refunds.

It might be trivial for you to add, but if you are too busy I can take a look as well.

[benmccann]

Pull requests welcome :-) You'll probably have to fix the XeroAPI-Schemas because they're horribly broken and generally need to be fixed for each new method call you want to add.

[rodgull]

I am adding credit note creation at the moment, but am running into the xsi:nil issue as detailed in Issue#6, so once I get it sorted I will put a pull request in. I have also done manual journals.