Import contribution policy from catarse
Refactor contribution policy
Remove payment_service_fee_paid_by_user from permited params in contribution policy
Migrate user contributions controller to pundit
Migrate project contributions to pundit
Remove cancan from users/contributions controller
Remove update action from projects/contributions controller
Remove unuset methods from contribution model
Refactor contribution spec messages
Merge pull request #157 from neighborly/jl-pundit-contributions

Migrate contributions to Pundit