Add static funciton to retrieve supported methods list.

[jedateach]

In payment 0.3 you can set the supported methods, but the only way you can get them is as form fields. That's not useful if you want to display the options in a drop-down list, or some other way.

[jedateach]

This is worknig, but currently requires some extra code like this:

    $supported_methods = PaymentProcessor::get_supported_methods();
    $source = array();
    if(count($supported_methods)){
        foreach ($supported_methods as $methodName) {
            $methodConfig = PaymentFactory::get_factory_config($methodName);
            $source[$methodName] = $methodConfig['title'];
        }
    }

..where $source contains the final list of supported methods.

This code could be factored into PaymentFactory, or similar.