Support for Bill entity

Hello ruckus,

I've added support for Bill entity. I was based on the previous created entities.
I also added some tests (using the sample XML from the Intuit Developer Reference).

As I am planning to add more support, I would like you to review my code (or give some feedback), if what I did follows your guidelines.

I also think there are some parts that could be generalized (such as PaymentMethodRef, InvoiceItemRef, CustomerRef).

I will look forward for your response!
Thank you very much!