Merge pull request #2404 from lorenzocaum/master

Fixed #2401 Added the country of Curaçao for Shopp 1.3
Cleaned up Shopp add-on docblock
Fixed add-on module loading issues affecting storage engines and the image server
Cleaned up PHP notices
Fixed PayPal Standard order processing
Corrected image request parameter signing and validation
Fixed #2403 Re-enabled report export headers
#2147 Added APC cache support for the WP load file/config file path

This will improve performance on hosting with APC support, but it has limited effect.
Cleaned up the docblock
#2289 Added the Advanced screen help tab
Removed PayPal Express Checkout from core to be moved to the PayPal Payments Pro add-on (in accordance with PayPal partner requirements)
#2289 Added Pages screen help tab
Fixed an issue with Shopp per-page theme templates
#2289 Added Images screen help tab
Added backwards compatibility support for non-prefixed gateway class settings
Fixed a problem detecting subscribed email error notifications that could cause fatal errors in early code
#1797 basic fix for Accounts > Downloads screen (remove duplicate dkeys in download URLs)
Closed #1797 dkeys are now unique, downloadable addons will be listed
#2275 updated further references to Customer to ShoppCustomer
#2275 updated further references to Order to ShoppOrder