Modifications to compra facil - Wsdl Url

Hello,

I have been testing the module and it seems to work quite nice with Multibanco.

I think it would be nice to be able to configure the $wsurl (web service url) variable from backoffice since going from test to production it is not necessary to edit the code (.module file), only changing in backoffice the variable.

It could be done using setting variable, i am attaching a patch that makes this simple change.

Best regards,
Joao Garin