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.
Changed the way the error message related to unsupported currency is displayed

The new logic displays the error message as an admin notice, like it's done when client_id and client_secret are missing. This allows to inform the user that the plugin won't work, without preventing him from configuring it. Note: the gateway will still be unavailable for payment, as it was done before.
Corrected comment
Merge pull request #2 from daigo75/master

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