Ordway

- Added support for updating following entities:
* Products
* Customers
* Plans
* Coupons
* Subscriptions
* Orders
* Webhooks
* BillingSchedules
* RevenueRules
* Invoices
* Payments
* PaymentRuns
* Credits

- Bug fixes
* Passing no `sort` argument to `ListAPIMixin.all` to `ListAPIMixin.list` caused an Ordway API error.
* Entities added in 0.2.0 were not instantiated on the `ordway.Client`, rendering them unusable.

- Added support for staging enviornment by passing `staging=True` to `ordway.client.OrdwayClient`

- Added new entities:
* JournalEntries
* PaymentRuns
* Statements
* Coupons
* Orders
* Usages
* BillingRuns
* RevenueSchedules
* BillingSchedules
* RevenueRules
* ChartOfAccounts
- Bug fixes
* Pass page size from `ListAPIMixin.all` to `ListAPIMixin.list`
* Unnecessary `GeneratorExit` exception

- Set Accept header to *application/json* by default for `ordway.client.OrdwayClient`'s session
- Attached a custom `requests.adapters.HTTPAdapter` to handle retries and HTTP timeouts
- Functionality for using a context manager with `ordway.client.OrdwayClient` added