added Invoices API:
  stripe-get-invoice
  stripe-get-invoice-lines
  stripe-new-invoice
  stripe-pay-invoice
  stripe-update-invoice
  stripe-delete-invoice
  stripe-get-invoices
  stripe-get-upcoming-invoices

added Invoice Items API:
  stripe-new-invoiceitem
  stripe-get-invoiceitem
  stripe-update-invoiceitem
  stripe-delete-invoiceitem
  stripe-get-invoiceitems