Added referral_code to Order

Also added validation (alphanumeric), but we're fairly lax about
whitespace and case, and clean up for them if they make minor errors.
added referral_code to order form/checkout code
moved admin order search into a model method
Added referral code search to admin forms
Added tests for admin order controller
Fixed up db/schema to remove spurious payments table
add referral code to order admin table
Merge pull request #318 from Skud/order_referral_code

Order referral code