Shopper Activity support and general improvements

• Deleted the beta Purchases method
• Added support for all Shopper Activity API functions
  • Includes a check for required fields and fails if not present
• Introduced a simpler approach for passing payloads to methods. The new structure only requires either an object for individual POST endpoints or an array of objects for batch endpoints. Prior to this change the complete payload schema would need to be passed to each method which wasn't obvious.
• Content-Type header is now application/json
• Minor style clean up

Complete API coverage and new Orders methods

• Deprecated the beta Purchases methods
• createPurchase now sends requests to Drip's order creation endpoint
• Complete API coverage