Moved checks to ensure that gateway is valid for use to a later stage

Originally, checks were performed during class initialisation. This was premature, for two reasons:
- Original check was setting "enabled" to false if plugin was not configured correctly. However, "enabled" and "usable" are two different concepts. When a gateway is enabled, it's enabled even when it's misconfigured.
- WooCommerce checks if a gateway is usable by calling object->is_available(). That's when it's important to verify that settings are correct.