New features: Orders API
* `CreateOrder` endpoint creates an itemized order which can be referenced in messages to the `Charge` endpoint.
* `BatchRetrieveOrders` retrieves order objects referenced in one or more transactions based on the provided `order_id`.
**Note:** at this point, `BatchRetrieveOrders only` returns information on paid orders made through Square's eCommerce APIs (Transactions, Checkout).
Other Changes
* `order` is removed from Transaction
* `order_id` is added to Transaction
* `OrderLineItemDiscountType.UNKOWN` renamed to `OrderLineItemDiscountType.UNKOWN_DISCOUNT`
* `OrderLineItemTaxType.UNKOWN` renamed to `OrderLineItemDiscountType.UNKOWN_TAX`
* `ChargeRequest.idempotency_key` is restricted to max length 192
* `ChargeRequest.card_nonce` is restricted to max length 192
* `ChargeRequest.customer_card_id` is restricted to max length 192
* `ChargeRequest.reference_id` is restricted to max length 40
* `ChargeRequest.note` is restricted to max length 60
* `ChargeRequest.customer_id` is restricted to max length 50
* `CreateCheckoutRequest.redirect_url` is restricted to max length 800
* Added `phone_number` and `business_name` to Location