Add store method to InvoiceBuilder::Client to render HTML invoices from a json statement and store them to S3.
Add a docstring for the store method.
Update README.
Update README.md
Merge pull request #2 from heroku/store_invoice

Add store method to InvoiceBuilder::Client. The store endpoint on InvoiceBuilder will render an HTML invoice from the POSTed JSON statement and store it to S3.