Error WC_Gateway_Wirecard class not found

Hello! I'm try to use your plugin with more than 5 payments gateways. In the moment I activate the plugin hive me an error because can't find the "WC_Gateway_Wirecard" class. I think this happened because the "add_payment_options" running before load all the gateways. So I add the priority 99 to be sure this will run after all the gateways are loaded. After this change, all works perfect.

Great plugin :)
Merge pull request #22 from gabrielperezs/patch-1

Error WC_Gateway_Wirecard class not found