Sort Payment gateway dropdown menu

At this moment payment gateways show up in the order they are installed. Shipping methods however are sorted.
It would be better to use SORT_NATURAL to have iDEAL show up at the right spot too but that seems to be a PHP 5.4 option.
Closed #3054 Use plugins_url() to allow domain mapping
Merge branch 'issue/3054'
Merge pull request #3047 from Just-m/JM-payment-order

Sort Payment gateway dropdown menu
Merge branch '1.3.x' of code.shopp.net:core/shopp into 1.3.x
Fixed #3053 Correctly process registrations, auto-login and redirect
Merge branch 'issue/3053'