Add prerequisite_customer_ids to PriceRule model

Add prerequisite_customer_ids to PriceRule model (#46)

- Recently added to Shopify's price rule API
  - https://help.shopify.com/api/reference/pricerule#prerequisite-customer-ids-property
- Add property to the model and fix up PriceRuleTest to accommodate the change

Add customer endpoint to customer saved searches

Add customers endpoint to customer saved searches (#45)

- Added dates to the customer saved search model
- Added function to customer saved search service that grabs all customers that apply to the customer saved search ID
  - https://help.shopify.com/api/reference/customersavedsearch#other
- Added tests for customer and customer saved search to ensure accurate param types
- Added createFromArray function to customer service for easier testing

Fix DiscountCode api calls

Fix discount code retrieval and model (#44)

- Corrected discount code model types to be consistent with types returned from the API
- Refactor discount code service to correct function for retrieving discount codes by price rule ID
  - Originally it assumed that one discount code was always returned but that endpoint can return multiple per price rule ID
  - Was also using wrong index on the `$raw` param for the above reason
  - Corrected such that it returns a collection of `discount_codes`
- Added function for retrieving singular discount codes by price rule ID + discount code ID
- Added test to ensure accuracy of serializing and de-serializing

Fix DiscountCode attribute mismatch

1.4.3

Fix usageCount setter (#43)

ApiSleeper implements RequestHookInterface

1.4.2

ShopifyApiHandler implements RequestHookInterface (#42)

Fix tests for removing install sleeper

InstallAndLogin needed the ApiSleeper removed (#41)

Since the install flow has not yet made any api calls, there is
no concern about hitting the api call limit. So rather than
switching the ApiSleeperContract over to the RequestHookInterface,
we just removed this usage of it.

Remove APISleeper from Install flow

InstallAndLogin needed the ApiSleeper removed (#40)

Since the install flow has not yet made any api calls, there is
no concern about hitting the api call limit. So rather than
switching the ApiSleeperContract over to the RequestHookInterface,
we just removed this usage of it.

Add Collects

Add support for Collects

Add Discount Code Endpoints

1.3.4

Feature/discount code and price rules functionality added (#37)

Add Image Attachment to model

1.3.3

Adding attachment to the Image model (#38)