Refactor configuration of plugin

Change the "shopify.php" into a bootstrap.php that calls Configure::write()
explicitly. This allows easy overriding of the configuration by just calling
Configure::write() manually in the main app's bootstrap.php.

Add the prefix "Shopify." to all configuration items to prevent namespace
collisions in the configuration.

Add an additional configuration parameter of "domain" to support private apps
that only act on a single domain.