add invoice create and invoice pay.
Merge branch 'master' into add-invoice-create-and-pay
add test
Add new invoice functions to README.
Merge pull request #1 from willwhite/add-invoice-create-and-pay

Add new invoice functions to README.
Merge pull request #51 from pchw/add-invoice-create-and-pay

Implements 'Create an invoice' and 'Pay an invoice'