This updates the repo to v0.1.20 of BTCQBO, which has the following improvements:
1. Better user experience. Flow through the sync/setup steps is more natural for the user.
2. Added email functionality. The QBO API does not allow issuance of receipts for invoice payments not made through Intuit's own payments module. The plugin now works around this issue by sending its own email receipts to customers when they pay an invoice.
