Check if gateways key exists and it is an array, to fix the following issue:

[Fri Jan 31 13:29:17 2014] [error] [client 80.69.69.100] FastCGI: server "/var/run/php-fpm/php54/php-cgi" stderr: PHP message: PHP Warning:  Invalid argument supplied for foreach() in /wp-content/plugins/wp-pronamic-ideal-develop/views/config-edit.php on line 593, referer: /wp-admin/edit.php?post_type=pronamic_gateway
Tweak - Reduced transient name length to avoid "data too long for column 'option_name'" errors.
Added support for Membership coupon codes.
Simplified use of filter.
Removed trailing space.
Redirect after payment status update to the registration completed page.
Updated change log.
Tweak - Easy Digital Downloads - Don't display gateway input fieldset if the gateway doesn't require input.
Updated the inline documentation of the iDEAL Basic date helper class on alphanumerical values.
Allow empty value for license key.
Fixed link to the settings page.
Getting ready for version 2.6.1.
Merge branch 'release/2.6.1'