* Maintain exception bubbling via PHPDoc `throws` tag.
* Consolidate sub-resources under their parent `Buyer` and `Refund` are sub-resources of `Invoice`, thus it is pragmatic to implement and test them within their parent's context.
* Relocate Rates resource actions trait to correct directory.
* Deprecate `Item()` in favor of `BillItem()` Discourage ambiguity of a BitPay `Item` where Bills and Subscriptions are concerned.
* Define commonly used BitPay constants, primarily to prevent typos.
* Subscriptions resource implementation.
* Update README