Create recurring donations model + Update controller action to create new recurring donation instance

[winsonwan]

1. create model, controller, and other miscellaneous default files (eg. spec files) for recurring donations using 'rails generate ...'
2. create recurring donations table and add references to related tables
3. add associations to related models
4. create recurring donations model method to create new recurring donations row from order object
5. modify store_controller#process_donation to create new recurring donation row when appropriate (and delete on failure)
6. change donation_type to donation_frequency in /donate view for clarity