Easy Digital Downloads - Per Product Emails 1.1.8

Improvement: The extension setting is now at Downloads > Settings > Emails > Purchase Receipts.
Dev: Update how licensing is instantiated.
Dev: Refactor how the plugin is booted and checks for requirements.

Fix: The integration with Software Licensing was using a deprecated method to get the license key.

Fix: Prevent a PHP error when no product IDs that need custom emails are found.
Fix: Allow attachments to be included when the EDD_Emails class is used.