perform recurring donation transaction

[winsonwan]

1. change 'donation_type' variable name in /donate view to 'donation_frequency' for clarity and consistency
2. create RecurringDonation model and update existing models to handle association with RecurringDonation
3. update store_controller#process_donation handle donation_frequency param
4. add new method to create and add recurring donation record to order (order#add_recurring_donation)

[winsonwan]

lgtm