change current_user to spree_current_user
Commit includes:
1 - Introducing credit reasons in separate table for reducing redundancy, duplications, and more effective recognizable store credits.
2 - Introducing expiry for credits using expiry:datetime column. Which can be nil for a never expiring store credit.
3 - Modified GiveStoreCredit Promotion Action for specific reason and expiring days, set days to 0 if credit never expires in Admin panel.
4 - Added a scope: active to store credits, which just loads active store credits.
5 - Modified engine to a separate engine.rb.