WTI
Update zeus
Use possibly from rubygems
Merge pull request #400 from sharetribe/update-possibly

Use possibly from rubygems
Cleaner payment classes
Move methods to CheckoutPayment
Payment belongs to PaymentGateway
Fix broken migration
Test cleanup
Remove new listing seller commission
Improve JS calculations
Payment gateway, total commission
Calculate gateway fee
Calculations
Seller commission
Fix minor issu in payment relations