(BETA) multi-subs support
Per-sub invoices and invoice items
Per-sub discounts
Take multi-subs out of beta

- Use standard API key in multi-subs tests
- Remove 'BETA' comments from new fields/methods
- Add 'legacy' comments to old fields/methods
Fixup API_BASE -> getApiBase bitrot in Subscription

Was changed in cbdda9a7255a6a160c212074f3562489df6558c9
Merge pull request #63 from jimdanz/multi-subs

Multi subs