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.