Fine tuned pmpro_gateways filter

The changes done in this pull request are summarized below.

There is no need to declare a global pmpro_gateways variable to get the list of available gateways. This has been moved to the pmpro_gateways function. 
So to get the list of available gateway just call the pmpro_gateways function.

Initially the pmpro_gateways filter doesn't work because the list of available gateway isn't returned, this has been fixed. To add a new payment gateway to the list of available gateway this is how to do it
```
function tbz_pmpro_gateways($gateways)
{
	if(empty($gateways['new_gateway'])){
		$gateways['new_gateway'] = __('New Gateway', 'pmpro');
    }
	return $gateways;
}
add_filter('pmpro_gateways', 'tbz_pmpro_gateways');
```
All reference to the global pmpro_gateways variable has been changed to the pmpro_gateways function.

Also the gateway classes for the payment gateways that comes with PMP are no more being added using the foreach statement, they are being added by including the files directly.