Allow gateway setting forms to be methods of a class

The current gateway helper code calls the gateway's settings form function directly, using the syntax

$var();

This works fine if the variable is a string containing a function name. If however, the variable is a class and method, in standard call_user_func() format, then this works on PHP 5.4, but not on earlier versions. The attached PR switches to using call_user_func() so that either syntax can be used.