Pull request thread for addressing TODOs

Making this a PR so we have some visibility into our progress. 

Outstanding TODOs: 
- [ ] app/assets/javascripts/credit_cards.js.coffee:19 (async credit card submission)
- [ ] app/controllers/credit_cards_controller.rb:16 (more specific credit card errors on create)
- [ ] app/controllers/credit_cards_controller.rb:27 (how best to render newly created credit cards?)
- [ ] app/controllers/events_controller.rb:37 (validate event creation cost field)
- [ ] app/controllers/payments_controller.rb:14 (use current_user.payment.build)
- [ ] app/controllers/payments_controller.rb:23 (present available information from Stripe::CardError to user)
- [ ] app/models/user.rb:66 (validate that amount charged is in cents, not dollars)
- [ ] app/models/user.rb:161 (how best to handle payments that have been made for events that were later deleted?)
- [ ] app/views/payments/index.html.erb:19 (extract payment views out of the payments index)
- [ ] app/views/payments/index.html.erb:24 (async payment submissions)
- [ ] app/views/payments/index.html.erb:45 (extract payment views out of the payments index)
- [ ] app/views/payments/index.html.erb:50 (async payment submissions)