Invoiced

- Add endpoints for managing payment sources (cards and bank accounts)

- Minimum version of Python supported is 3.4
- Added connect and read timeouts to HTTP requests

New objects:
- Coupons
- Letters
- Notes
- Tasks
- Tax Rates
- Text Messages

New endpoints:
- Consolidate invoices
- Initiate a charge
- Preview subscriptions
- Void invoices, credit notes, and estimates

- Added support for idempotency keys with `idempotency_key` option on requests

- Added `RateLimitError` class to catch rate limiting errors

- Added `CatalogItem` object
- Added `Plan` object

- Added `PaymentPlan` object