Refactory data model
Fix pep8
Remove unused status of invoice, rename money fields all to `amount` (for consistency)
Implement invoice refund
Implement invoice cancel
Add a test to ensure canceld subscription will not yield new invoices
Fix invoice.transactions has no order_by issue
Fix invoice.transactions has no order_by issue
Add missing sphinx in document requirements.txt
Implement missing invoice listing of subscription view
Fix integration tests
Update getting started document page
Add invoice listing under plan
Update documents for invoice
Update documents, add updating invoice instrument URI part
Add some missing POST in documents
Merge branch 'new-model' into invoicing