Allow creation of new Invoice off Customer

The ```Customer``` object has the ability to view an upcoming invoice through ```.upcoming_invoice```. However, the ```Invoice``` that gets returned is only a preview of what the invoice will look like.  In other words, the upcoming invoice does not have an invoice id and does not actually exist yet.

This means that if I want to create a new invoice immediately for a customer, I have to turn back to the ```Invoice``` class, passing in my customer and token again.

It would be much nicer just to be able to create a new invoice off the customer itself.  This makes a lot of sense given that it is already possible to create invoice items off a customer, view an upcoming invoice, but no way to immediately create a new invoice.